Apparatus, systems and methods for use in three-dimensional printing

ABSTRACT

Apparatus, systems and methods for use in three-dimensional printing are shown and described. Various embodiments of the invention allow for more precise and controlled delivery of heat to achieve interlayer drying; isolation of the working region from the outside for reasons of cleanliness and in connection with the vapors of organic solvents; better control of the temperature of the working region; better accuracy in the flowrates of binder fluid dispensed; matching of delivered flowrates for multiple dispensers; verification of delivered flowrate or drops; provision for easier changeover of the machine from one powder to another; cleanability; and other needs.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed toward apparatus, systems andmethods for use in three-dimensional printing.

[0003] 2. Description of the Related Art

[0004] Three-dimensional printing is a process of depositing successivelayers of powder onto a substrate, and causing portions of each layer toadhere or bind to themselves and to adjacent earlier-deposited layersthrough the action of a liquid called a binder. Through this sequentialprocess a three-dimensional product is ultimately formed. Because of thesequential nature of this process, products having complex shapes andeven interlocking parts can be formed, products that are otherwiseextremely difficult if not impossible to fabricate through traditionalmeans.

[0005] Powder has typically been deposited through one of two methods.One method has been to spread dry powder with a roller across the top ofthe substrate or prior layer. This method is suited to creating powderlayers with thicknesses of around 0.005 inch (127 microns) or greater,and to powders whose average particle size is greater than a minimumvalue which is somewhere in the range of 5 to 20 microns.

[0006] The other method of depositing powder has been to deposit layersof a powder-carrying slurry onto the substrate or prior layer. Theslurry comprises a carrier liquid with a fairly high content ofsuspended solid particles. The slurry carrier liquid may be formulatedto encourage the particles to remain suspended and to discourage themfrom agglomerating with each other. A discharge of slurry (usuallycontinuous) from a nozzle is rastered across the bed. Slurry depositionhas typically been used for depositing rather thin layers of powderwhose particle size is finer than is practical with roller-spreading ofpowder. When the slurry has been deposited, all or most of the slurrycarrier liquid must be removed from the deposited layer before printingof binder liquid. This has been accomplished through some combination ofpercolation, natural evaporation and evaporation due to externallyapplied heat.

[0007] After each layer of powder has been deposited on the build bed,by either the roller method or the slurry method discussed above, aprinthead dispenses binder or another fluid onto preselected areas ofthe powder to form a layer of the final product. The binding action canbe achieved through dissolution of powder by the binder followed byresolidification when the binder evaporates, or through an adhesivewhich is initially dissolved in or mixed with the binder and is leftbehind on the powder when the volatile component(s) of the binderevaporate. Typically the binder is selected to have an appropriatedegree of volatility so that its volatile component(s) evaporate,leaving behind solidified powder, after a desired amount of time.

[0008] The printhead can operate in a raster mode, which is similar toan inkjet printer, progressively making rapid, lateral passes across thebuild bed, moving slightly forward with each pass, until it hasdispensed fluid along the entire length of the build bed. The lateraldirection of the printhead has been referred to as the fast axis, as theprinthead moves along this axis relatively quickly. The longitudinaldirection, perpendicular to the fast axis, has been referred to as theslow axis, because movement along the slow axis does not occur duringfast axis movement and in general is slower. It has also been possibleto perform vector printing, in which there is simultaneous motion inboth horizontal axes to enable the printhead to move in curved paths. Ithas also been possible to use both raster printing and vector printingin a print job.

[0009] Controlling the duration of drying and the extent of dryingbefore application of the next layer of powder has been importantbecause these influence bleeding (i.e., the spreading of dispensedliquid in the powder before it dries, which affects dimensions and thesurface finish of a part) and because they affect the adhesion betweenlayers (i.e., the strength of the printed part).

[0010] Analyzing both the rate and the extent of drying betweensuccessive layers has indicated that there is an optimal range for eachof these parameters. The evaporation rate of a given binder at theoperating temperature, which is typically room temperature, may or maynot be within that optimal range. Presently, one design variableavailable to influence drying rate is the formulation of the binder.Another available variable is the temperature of the print bed, whichimpacts the entire machine design and process. Yet another variable isthe length of time between depositing layers, which affects the extentof drying between layers but does not affect the rate of drying andhence does not control bleeding.

[0011] Heating during portions of the three-dimensional printing processis known in crude forms. For example, externally applied heat has beenused for purposes of evaporating slurry carrier liquid. However, thisexternal application of heat to a slurry-deposited layer, which hastypically occurred prior to printing of binder onto that layer, hascaused evaporation of slurry carrier liquid. This is not the same ascausing the evaporation of binder liquid, which occurs after printing ofbinder onto that layer.

[0012] Heating of the build bed to accelerate evaporation of thevolatile part of the binder liquid has been proposed, such as in U.S.Pat. No. 5,204,055, but using fixed-place heat sources locatedsufficiently far away from the build bed to allow room for all the otherequipment is not very precise in the application of heat directly to thebed, or in uniformity of delivery of heat to all places of the bed, andin such a manner as to achieve a desired remaining saturation of theprinted portions of the layer after completion of the interlayer drying.For example, in construction of parts by three-dimensional printing,some layers may have large areas printed upon by binder liquid whileanother layer may have only small areas printed upon by binder liquid.Treating all of these layers the same as far as application ofinterlayer drying heat produces results which differ between thesesituations as far as what is the saturation of the bed at the time ofthe spreading of the next layer of powder and subsequent printing uponit.

[0013] Existing three-dimensional printing machines have not beensufficiently cleanable to be useful for medical manufacturing purposes.Likewise, refilling, replacing and/or changing the powder in existingthree-dimensional printing machine has been a difficult job and couldspread powder throughout the machine, requiring additional cleaning and,possibly, disinfecting. Much of the process of loading and unloadingpowder as well as cleaning has involved the entire machine, making theentire machine unavailable for any other purpose during that time.

[0014] For some purposes it may be desirable to use binder liquids,which comprise organic solvents. Such solvents may be particularlydesirable for use in certain medical applications, which may requiredissolution of substances not soluble in water-based liquids. Organicsolvents may have hazards not found with water-based liquids. Forexample, chloroform, an important organic solvent for purposes ofmedical interest, is a suspected human carcinogen. Many other organicsolvents of interest, such as ethanol and other alcohols, and acetone,in addition to being in some cases toxic, are flammable. Flammablesubstances can be explosion hazards for certain concentrations of theirvapors in air or certain vapor/oxygen ratios. Existing three-dimensionalprinting machines intended for more ordinary binders, usually aqueousbinders, have not had features appropriate to adequately deal with thesehazards.

[0015] Existing three-dimensional printing machines may have had anenclosure designed to prevent objects or operators' body parts fromaccidentally entering into the workspace. However, they have not had anenclosure sufficient to maintain an environment suitable for processingmedical and pharmaceutical products and devices.

[0016] Existing three-dimensional printing machines may have heat loadswithout having good ways of managing and removing that heat. It can beimportant to maintain well-controlled temperature of components near theworking region, because for example the dispensing of binder liquid cansuffer from irregularities if there are significant variations in thetemperature of the liquid as it is dispensed.

[0017] Existing three-dimensional printing machines typically haveallowed manual changeover from one powder to another and associatedcleaning procedures, but not in a quick or easy manner.

[0018] Precision in determining dispensed fluid volume has traditionallynot been critical. Existing three-dimensional printing machines havedispensed binder at flowrates which are known to an accuracy suitablefor industrial purposes, but not to an accuracy suitable for demandingmanufacturing such as manufacturing of medical articles. Furthermore, inexisting three-dimensional printing machines there has been no way ofmeasuring the actual delivered flowrate during printing or evenverifying the delivery of a drop at any given location where delivery ofa drop was commanded.

[0019] Thus, prior to the present invention there existed a need formore precise and controlled delivery of heat to achieve interlayerdrying; isolation of the working region from the outside for reasons ofcleanliness and in connection with the vapors of organic solvents;better control of the temperature of the working region; better accuracyin the flowrates of binder fluid dispensed; matching of deliveredflowrate for multiple dispensers; verification of delivered flowrate ordrops; provision for easier changeover of the machine from one powder toanother; cleanability; and other needs.

BRIEF SUMMARY OF THE INVENTION

[0020] The present invention is directed toward apparatus, systems andmethods for use in three-dimensional printing of products.

[0021] Embodiments of the present invention may incorporate a powderlayer depositor, a printhead for dispensing binder or other fluids, andan interlayer dryer heat source for controlling the rate and extent towhich volatile liquid evaporates from the powder. In one particularembodiment, a roller deposits a layer of powder onto a build bed, theprinthead dispenses onto the powder a fluid comprising a volatileliquid, and the interlayer dryer heat source controllably acceleratesevaporation of the volatile liquid. Embodiments of the invention controlthe heater based on time, the temperature of the powder, the moisturecontent in the powder, empirical data, or a combination of these and/orother factors. By controlling the rate and extent of evaporation of thefluid in the powder, the inventors have been able to improve the qualityof products fabricated through the three-dimensional printing process.

[0022] Embodiments of the present invention are also directed towardthree-dimensional printing machines incorporating a roller that isheated and/or maintained at a pre-selected temperature. One particularembodiment of the invention incorporates a roller controllable to spreada powder onto a build bed, and a roller heater suitable to heat theroller to an elevated temperature. The roller heater can be internal toor external of the roller, and can be controlled based on thetemperature of the roller surface, the temperature of the internalcavity of the roller, the temperature or moisture content of the powder,or a combination of these and/or other factors. The present invention isalso directed toward methods of heating or maintaining a desired rollertemperature.

[0023] Embodiments of the present invention are also directed towardseveral means of enclosing and controlling the environment surroundingthe three-dimensional printing machine. Particular embodiments of theinvention incorporate an enclosure, and/or a ventilation, recirculationand air purification system and/or a system for maintaining the interiorof the enclosure at a desired pressure, which can be at, above or belowthe pressure of the surrounding environment. With such embodiments, theair surrounding the printing machine may be maintained at a desiredtemperature, at a pressure greater than the surrounding environment toprevent contaminants from entering the enclosure, at a pressure lowerthan the surrounding environment, at a substantially stationary state toavoid disruption of droplet trajectories, and/or free from contaminantsand harmful fumes. Gases other than air can be supplied to the enclosureinterior.

[0024] Embodiments of the present invention are also directed towardsystems and methods for precisely controlling the amount of fluiddeposited onto the product, which can be desirable for medical,pharmaceutical and other applications. Particular embodiments of thepresent invention can be used to test, group and calibrate microvalvesso that the amount of fluid dispensed from each dispenser in amulti-dispenser printhead deposits a substantially identical volume offluid as the other dispensers in the printhead. In addition, particularembodiments of the present invention—including particular actuationpulse waves used to open and close each microvalve independently—can beused to dispense a precise, measurable and repeatable volume of fluidfrom each dispenser, from one cycle to the next and from one product tothe next. Embodiments of the present invention are also directed towardsizing of valves relative to the viscosity of the fluid, and towardmeasuring delivered amounts of fluid or verifying the delivery of dropswhen commanded.

[0025] Embodiments of the present invention are also directed towardmeasuring the flowrate of fluid that is being dispensed or verifyingthat fluid actually is dispensed when intended.

[0026] The present invention is also directed toward several means ofbuilding, controlling, and maintaining a three-dimensional printingapparatus and system.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0027]FIG. 1 is an isometric view of a three-dimensional printing systemaccording to an embodiment of the present invention, viewed fromslightly below an operating plane at which powder spreading and printingtake place.

[0028]FIG. 2 is an enlarged isometric view of a portion of thethree-dimensional printing system of FIG. 1, viewed from slightly abovethe operating plane.

[0029]FIG. 3 is an elevation view of the three-dimensional printingsystem of FIG. 1.

[0030]FIG. 4 is a left end view of the three-dimensional printing systemof FIG. 1, from which background structure may have been removed forclarity.

[0031]FIG. 5 is a right end view of the three-dimensional printingsystem of FIG. 1, from which background structure may have been removedfor clarity.

[0032]FIG. 6 is a plan view of the three-dimensional printing system ofFIG. 1.

[0033]FIG. 7 is an elevation view of a powder bed assembly andpositioning system from the three-dimensional printing system of FIG. 1,the upper portion of which is illustrated in cross-section.

[0034]FIG. 8 is a plan view of the powder bed assembly of FIG. 7.

[0035]FIG. 9 is a cross-sectional elevation view of the powder bedassembly of FIG. 7, viewed along Section 9-9 of FIG. 8.

[0036]FIG. 9A is a magnified view of a portion of the powder bedassembly of FIG. 9.

[0037]FIG. 10 is a cross-sectional elevation view of the powder bedassembly of FIG. 7, viewed along Section 10-10 of FIG. 8.

[0038]FIG. 11 is an elevation view of another powder bed assembly andpositioning system from the three-dimensional printing system of FIG. 1,the upper portion of which is illustrated in cross-section.

[0039]FIG. 12 is a plan view of the powder bed assembly of FIG. 11.

[0040]FIG. 13 is a cross-sectional elevation view of the powder bedassembly of FIG. 11, viewed along Section 13-13 of FIG. 12.

[0041]FIG. 14 is a cross-sectional elevation view of the powder bed ofFIG. 11, viewed along Section 14-14 of FIG. 12.

[0042]FIG. 15 is a plan view of a tray subassembly from the powder bedassembly of FIG. 11.

[0043]FIG. 16 is a cross-sectional elevation view of the carrier platecontained in the tray subassembly of FIG. 15, viewed along Section16-16.

[0044]FIG. 17 is an elevation view of the carrier plate of FIG. 15,illustrating a gasket.

[0045]FIG. 18 is an isometric view schematically illustrating the powderbed assembly of FIG. 11.

[0046]FIG. 19 is an isometric view illustrating the tray subassemblyremoved from the powder bed assembly of FIG. 11.

[0047]FIG. 20 is a left end view of a roller assembly from thethree-dimensional printing system of FIG. 1.

[0048]FIG. 21 is an elevation view of the roller assembly of FIG. 20.

[0049]FIG. 22 is an enlarged left end view of a portion of the rollerassembly of FIG. 20.

[0050]FIG. 23 is a right end view of the portion of the roller assemblyof FIG. 22.

[0051]FIG. 24 is an isometric view of a printhead of thethree-dimensional printing system of FIG. 1.

[0052]FIG. 25 is an exploded isometric view of the printhead of FIG. 24.

[0053]FIG. 26 is an elevation view of the printhead of FIG. 24.

[0054]FIG. 27 is a right end view of the printhead of FIG. 24.

[0055]FIG. 28 is an elevation view of a portion of the printhead of FIG.24, viewed along Section 28-28 of FIG. 27.

[0056]FIG. 29 is an elevation view of a supply assembly from theprinthead of FIG. 24, shown in partial cross-section.

[0057]FIG. 30 is a cross-sectional view of the supply assembly of FIG.29, viewed along Section 30-30.

[0058]FIG. 31 is a cross-sectional view of the supply assembly of FIG.29, viewed along Section 31-31.

[0059]FIG. 32 is an elevation view of a valve from the printhead of FIG.24.

[0060]FIG. 33 is a right end view of the valve of FIG. 32.

[0061]FIG. 34 is a cross-sectional elevation view of the valve of FIG.32, viewed along Section 34-34 of FIG. 33.

[0062]FIG. 35 is an elevation view of a fin member from the printhead ofFIG. 24.

[0063]FIG. 36 is a plan view of the fin member of FIG. 35.

[0064]FIG. 37 is a side view of the fin member of FIG. 35.

[0065]FIG. 38 is a plan view of an orifice plate and orifice tube fromthe printhead of FIG. 24, shown from below the printhead as oriented inFIG. 26.

[0066]FIG. 39 is a cross-sectional view of the orifice plate and orificetube of FIG. 38, viewed along Section 39-39.

[0067]FIG. 39A is an enlargement of a portion of FIG. 39.

[0068]FIG. 40 is graph plotting the drop mass versus the pulse width offluid dispensed from a printhead such as that illustrated in FIG. 24.

[0069]FIG. 41 is a plan view of a heater of the three-dimensionalprinting system of FIG. 1.

[0070]FIG. 42 is a cross-sectional elevation view of the heater of FIG.41, viewed along Section 42-42.

[0071]FIG. 43 is a cross-sectional elevation view of the heater of FIG.41, viewed along Section 43-43.

[0072]FIG. 44 is an elevation view of a heater tube from the heater ofFIG. 41.

[0073]FIG. 45 is an axial cross-section of the heater tube of FIG. 44.

[0074]FIG. 46 is an elevation view of an enclosed three-dimensionalprinting machine according to another embodiment of the presentinvention.

[0075]FIG. 47 is a left end view of the three-dimensional printingmachine of FIG. 46.

[0076]FIG. 48 is a right end view of the three-dimensional printingmachine of FIG. 46, illustrating access doors in both the open andclosed positions.

[0077]FIG. 49 is a top view of a lower portion of the three-dimensionalprinting machine of FIG. 46.

[0078]FIG. 50 is an elevation view of an enclosed three-dimensionalprinting machine with auxiliary equipment mounted on carts that areseparate from the three-dimensional printing machine itself according tostill another embodiment of the present invention.

[0079]FIG. 51 is an isometric view schematically illustrating athree-dimensional printing machine incorporating a roller heateraccording to an embodiment of the present invention.

[0080]FIG. 52 is an elevation view of the three-dimensional printingmachine of FIG. 51.

[0081]FIGS. 53 and 54 collectively illustrate a fluid introduction,storage and supply system according to one embodiment of the presentinvention.

[0082]FIG. 55 illustrates an in-line flowmeter according to anembodiment of the present invention, using a differential pressuremeasurement.

[0083]FIG. 56 illustrates an in-line flowmeter according to anembodiment of the present invention, using a technique which tracks themotion of a gas bubble in a conduit.

DETAILED DESCRIPTION OF THE INVENTION

[0084] The present detailed description is generally directed towardsystems, apparatus and methods for three-dimensional printing systems.Specific details of certain embodiments of the invention are set forthin the following description and in FIGS. 1-56 to provide a thoroughunderstanding of the illustrated embodiments. One skilled in the art,however, will understand that the present invention may have additionalembodiments, and may be practiced without several of the detailsdescribed in the following description and illustrated in the Figures.

[0085] General Description of Illustrated Three-Dimensional PrintingSystem

[0086] FIGS. 1-6 illustrate a three-dimensional printing machine 100according to one particular embodiment of the present invention. Theillustrated three-dimensional printing machine 100 incorporates,generally, a first powder bed assembly and positioning system 200, asecond powder bed assembly and positioning system 300, a roller assembly400, a printhead assembly 500, and an interlayer drying heater assembly600. The structural elements of the three-dimensional printing machine100 generally consist of a lower support system 102, a table-top 104resting on the lower support system, and a gantry 106 resting on thetable-top 104 and/or on the lower support system.

[0087] The support system 102 is made up of a number of legs 108,lateral braces 110 extending between many of the legs, and feet 112, oneat the lower end of each leg. In the illustrated embodiment, the supportsystem 102 has six legs 108 arranged in a rectangular configuration, andthe lateral braces 110 extend between adjacent legs around the perimeterof the support system. The number and configuration of the legs 108 canvary dramatically without deviating from the spirit of the presentinvention. Likewise, after reviewing this disclosure, one of ordinaryskill in the art will appreciate that the three-dimensional printingmachine 100 can be supported in other equivalent manners.

[0088] The feet 112 at the lower end of the legs 108 may support andlevel the support system 102, and distribute the weight of thethree-dimensional printing machine 100. In addition, each foot 112 maybe independently and/or automatically adjustable to conform to theparticular foundation on which the three-dimensional printing machine100 rests. The inventors appreciate that the feet 112 could be replacedwith wheels, casters, skids or other structures without deviating fromthe spirit of the invention.

[0089] The configuration of the illustrated legs 108 defines a centralopening 114, inside which equipment, wiring, ducts or other objects canbe stored or positioned during use. The central opening 114 can bereadily accessible as illustrated in FIG. 1, or can be enclosed orcovered, as discussed in detail below. The illustrated table-top 104rests on the support system 102, and can be attached thereto withfasteners or other means generally understood in the art. Theillustrated table-top 104 has an upper surface 116, upon which otherstructural and functional elements of the three-dimensional printingmachine 100 can rest or be attached. An edge 118 of the table-top 104provides a surface to which items such as controls, a gutter fortrapping debris, or other features can be affixed. After reviewing thisdisclosure, one of ordinary skill in the art will appreciate that manyother features could be mounted to the edge 118 of the table-top 104.

[0090] Table-top 104 and in general any of the components describedherein could be coated with a coating, such as to enhance cleanability,which may be selected from the group consisting of: anodized finish,hardcoat anodized finish, siloxane, baked enamel, electroplated enamel,electroless nickel, coated elastomer, laminated elastomer, dipped latex,sprayed latex, polyvinyl chloride, nylon, polytetrafluoroethylene,polyurethane, an epoxy resin, or a polyester. A specific possibility forthe coating is the coating designated Endura 103, made by EnduraCoatings, Warren Mich., which is produced by a multi-step process thatcombines conversion of the aluminum surface to an aluminum oxideceramic, and the controlled infusion of sub-micron sized particles ofhigh temperature, low friction fluoropolymers. The coating becomes anintegral part of the surface base metal, as compared to a typicalsurface coating, and provides a combination of release (non-stick),non-wetting, uniform heat distribution, low friction, permanent drylubrication, chemical and corrosion resistance, and hardness properties.

[0091] The gantry 106 in the illustrated embodiment is fabricated fromfour upright structural members 120, a rectangular structural member 122positioned on the four upright structural members, and a shear brace 124between each of the upright members and a point along the perimeter ofthe rectangular structural member.

[0092] Attached to the gantry 106 are rails 130 along which a firstcarriage 126 can move, carrying the roller, assembly 400 lengthwisealong the three-dimensional printing machine 100. Similarly, a secondcarriage 128 can be operated to move, carrying the fast axis assemblyincluding the printhead assembly 500 along the length of thethree-dimensional printing machine 100. The movement of the secondcarriage 128 corresponds to movement of the printhead assembly in thedirection of the slow axis.

[0093] As best illustrated in FIGS. 3-6, the first and second carriages126, 128 ride along the rails 130 that extend along the length of thethree-dimensional printing machine 100 above the gantry 106. The rails130 are fixed with respect to the top of the gantry 106, and remainstationary as the first and second carriage 126, 128 move along them.

[0094] The first carriage 126 rides along the rails 130 on a flexiblebearing assembly 132 on one of the rails and a rigid bearing assembly134 on the other of the rails (FIG. 4). A lower portion of each bearingassembly 132, 134 rides on a corresponding one of the rails 130, and anupper portion of each is mounted to the first carriage 126. In theflexible bearing assembly 132, the upper and lower portions areconnected together by a vertical, thin web 136, while in the rigidbearing assembly 134 there is a thick web 138. The thin web 136 issufficiently flexible to allow the first carriage 126 to expand orcontract in the lateral direction, such as due to thermal expansion,without the carriage binding on the rail 130. That is, if the firstcarriage 126 expands or contracts in the lateral direction, the thin web136 flexes, preventing the carriage from exerting an excessive force onthe rails 130, which could otherwise prevent movement of the firstcarriage 126 along the rails.

[0095] A roller assembly driver 140 is mounted to the rectangularstructural member 122 of the gantry 106, and is linked to the firstcarriage 126 to controllably move the first carriage—and with it theroller—back and forth along the length of the three-dimensional printingmachine 100. In the illustrated embodiment, the roller assembly driver140 is a screw drive. The inventors appreciate that other, equivalentdrive mechanisms could be substituted for the screw drive withoutdeviating from the spirit of the invention. A rack and pinion, or aprecision ball screw, or other devices could be used. In particular, alinear motor could be used. In a linear motor, one of the twoelectromagnetically interacting components is stretched out in astraight configuration rather than being configured in a closed circleor loop as in a conventional rotary motor. Of the twoelectromagnetically interacting components, both could beelectromagnetic coils, or one could be an electromagnetic coil and theother could be a permanent magnet, or one could be an electromagneticcoil and the other could be an electrically conductive structure inwhich currents are induced. Motors could be servomotors having built-infeedback components. All of these comments apply as well to other drivesystems within the three dimensional printing machine.

[0096] The proximity of the roller assembly driver 140 to the rigidbearing assembly 134 and the lateral play in the roller assembly drivercombine to allow the rigid bearing assembly 134 to have a thick web 138.If the roller assembly driver 140 were positioned closer to the flexiblebearing assembly 132, the rigid bearing assembly 134 may need to bereplaced with a flexible bearing assembly 132. The inventors appreciatethat, having reviewed this disclosure, one of ordinary skill in the artwill understand the functioning of the rigid bearing assembly 134 andthe flexible bearing assembly 132, and will understand that variationscan be made to this particular structure without deviating from thespirit of the invention.

[0097]FIG. 5 illustrates the engagement between the second carriage 128and the gantry 106. The second carriage 128 operates in a mannergenerally the same as the first carriage 126. The second carriage 128,however, rides on a pair of flexible bearing assemblies 132, each ofwhich has a thin web 136. The inventors selected this particularconfiguration for the illustrated embodiment because, among otherthings, the second carriage 128 is rigidly mounted to a slow axis driver142, which is centrally located with respect to the lateral dimension ofthe gantry 106. The illustrated slow axis driver 142 is a precisionscrew drive that is closely controllable to move the printhead assembly500 in the direction of the slow axis to the precisely desired locationwith respect to the second powder bed assembly and positioning system300, and to move the printhead assembly horizontally toward and awayfrom the build bed 302. Because the printhead assembly driver 142 isrigidly mounted to a central location along the length of secondcarriage 128, lateral expansion of the second carriage may occur roughlyequally in opposing directions. Thus, in the illustrated embodiment theinventors have chosen to incorporate flexible bearing assemblies 132 onboth sides of the second carriage. As indicated above, however, one ofordinary skill in the art will appreciate that variations can be made tothe illustrated embodiment without deviating from the spirit of theinvention. In particular, a linear motor could be used.

[0098] Mounted to the underside of rectangular structural member 122 ofgantry 106 may be a thin sheet that occupies most of the region abovethe working region (i.e. the feed bed and build bed) in a manner whichdoes not interfere with the motion of first carriage 126 and secondcarriage 128. This thin sheet may serve to catch any debris, dust,contaminants etc. that might be generated by actions taking place abovethe sheet, including motions of the first and second carriages. Althoughsubstantial efforts will likely be made to achieve and maintain cleanoperation of the various machine components, it still may be helpful toguard against debris, dust and contaminants that might be generatedabove the working region.

[0099] A fast axis motor 144 drives a cable 146 (FIG. 6) in alternatingdirections to move the printhead assembly 500 back and forth along thefast axis above the build bed 302. Opposing ends of the cable 146 arewrapped in opposite directions around a drum 148 that is coupled to thefast axis motor 144. Consequently, when the drum 148 rotates, a firstportion of the drum feeds the cable 146 while a second portion of thedrum takes up the opposing end of the cable. When the drum 148 rotatesin the opposite direction, the first portion of the drum takes up thecable 146 and the second portion of the drum feeds the cable. Thus,rapid rotation of the drum 148 in alternating directions results inrapid movement of the printhead assembly 500 in opposing directionsalong the fast axis. The inventors appreciate that other, equivalentdrive mechanisms could be substituted for the motor and drum drivewithout deviating from the spirit of the invention. In particular, alinear motor could be used.

[0100] In the illustrated embodiment, the printhead assembly 500 ismounted on the second carriage 128 by an air bearing 150 (FIG. 5) toreduce friction between the printhead assembly and the second carriage.In addition, the second carriage 128 is fitted with an encoder strip 152(FIG. 5) over which the printhead assembly 500 moves. A reader 153 (FIG.5) in the printhead assembly 500 can determine the precise location ofthe printhead assembly with respect to the second carriage 128 and,thus, can determine the precise location of each of the nozzles ororifices 547 (FIG. 28) in the printhead assembly, based on the readingfrom the encoder strip 152. The printhead assembly 500 can use thisinformation to trigger a dispenser, as opposed to strictly triggeringthe dispenser on a time-basis. For example, counting hardware orsoftware can count how many encoder markings have been read since theprevious dispense command or since some other reference, and based onthat count can issue the next dispense command. By this technique, minorirregularities or non-uniformities in the velocity of the printheadassembly 500 along the fast axis of motion have no effect on the dropplacement accuracy, because drops are not dispensed based on thepresumed velocity of the printhead, but rather are dispensed based onactual position of the printhead. For example, if drops were dispensedbased on the known angular position of a stepper motor or servomotor, asis done in many inexpensive ink-jet printers, that would still allowpossible inaccuracies of drop delivery to occur based on slack,stretching, etc. of belts and cables which carry motion between themotor and the actual printhead. In the three-dimensional printingmachine 100 of the present invention, all dimensions may be much largerthan those of an ink jet printer, and so such errors could be magnified.Such errors can be eliminated by the use of an encoder strip.

[0101] Powder Bed Assemblies

[0102] The three dimensional printing machine 100 comprises a build bedin which the part is constructed. If powder is roller-spread, themachine may also comprise a feed bed that presents powder to a roller.Two different designs of powder bed are presented here, either of whichcould be used either as a feed bed or as a build bed. FIGS. 7-10illustrate the first type of powder bed assembly and positioning system200 of the present embodiment. The first powder bed assembly andpositioning system 200 is generally made up of a first powder bedassembly 202, a first powder bed piston assembly 204 and a first powderbed motion controller 206. The first powder bed assembly and positioningsystem 200 is illustrated in this embodiment being used as the feed bedassembly, although it should be understood that this design could beused in either location. In this design, the first powder bed assemblyand positioning system 200 is removable from the three-dimensionalprinting machine 100.

[0103] The first powder bed assembly 202 is mounted on the table-top 104at a pair of opposing powder bed assembly tracks or guides 208. In theillustrated embodiment, the powder bed assembly guides 208 are mountedto the table-top 104 by a number of fasteners 210 extending through thetable-top from below, and fastening to the powder bed guides. The firstpowder bed assembly 202 comprises a pair of opposing, lower walls 212spaced apart from each other to engage with a corresponding one of thepowder bed assembly guides 208, and the first powder bed assembly 202can thus slide along the powder bed assembly guide 208 into and out ofoperating position. When the first powder bed assembly 202 is in theoperating position, retention pins 214 can be inserted laterally throughthe powder bed assembly guide 208 and into the respective lower wall 212to retain the powder bed assembly in its operating position.

[0104] The first powder bed assembly and positioning system 200 can beconfigured with contact sensors and alarms to confirm when the firstpowder bed assembly 202 is in the proper position for operation, and tonotify the operator when the powder bed assembly is out of operatingposition. An individual of ordinary skill in the art, having reviewedthis disclosure, will appreciate the many equivalent means forconfiguring and operating such sensors and alarms.

[0105] A separator structure 216 of the first powder bed assembly 202divides the functioning, upper portion of the powder bed assembly fromthe structural, lower portion of the powder bed assembly. In the upperportion of the powder bed assembly, a perimeter wall 220 defines theperimeter of the functional portion of the first powder bed assembly202. During operation, the perimeter wall 220 retains powder within itsconfines. A carrier plate 222 is closely retained within the perimeterwall 220, and supports the powder from below. Carrier plate 222 may berectangular with rounded corners, or can have another shape. The carrierplate 222 is closely fitting with respect to the interior of perimeterwall 220 and is adapted to slide up and down in the perimeter wall 220to move powder up or down. A piston opening 218 in the separatorstructure 216 allows the feed bed piston assembly 204 to engage thecarrier plate 222 from below, while the separator structure stillretains the carrier plate 222 so that the carrier plate cannot slide outof the bottom of perimeter wall 220. A bottom surface 224 of the carrierplate 222 engages the first powder bed piston assembly 204 duringoperation.

[0106] As best illustrated in FIG. 9A, the perimeter of the carrierplate 222 is fitted with an external, polymeric gasket 226 and a pair ofinternal O-rings 228. The polymeric gasket 226 can slide along the innersurface of perimeter wall 220 to prevent powder from escaping from thepowder bed assembly 202 during use. The O-rings 228 can urge thepolymeric gasket 226 against the inner surface of perimeter wall 220.The inventors appreciate that, after reviewing the present disclosure,one of ordinary skill in the art will appreciate that there are other,equivalent configurations and materials, such as a felt gasket, can beused to prevent powder from passing between the carrier plate 222 andthe inner surface of perimeter wall 220. Likewise, the sealing meansdescribed in connection with the first powder bed assembly 202 can beinterchanged with that described later in connection with thealternatively designed second powder bed assembly 302.

[0107] The top edges of perimeter wall 220 may form the surfaces uponwhich may bear sliders as later described, to help confine powder aspowder is being rolled. Beyond the outer edges of the perimeter wall220, there may be intermittent gaps all around the outer edges of theperimeter wall 220. These gaps are places where powder can drop into soas to be removed from the immediate work area during operation. On thesides, powder which spills out sideways during the rolling-spreadingoperation and which is not contained by the sliders 432 (FIG. 20) canfall off into those recesses. A still further outer siderail 250 may beprovided which may bear rollers casters etc. such as those from theheater described later. The top surface of the illustrated perimeterwall 220 is a horizontal surface and so are the top surfaces of theillustrated siderails 250. The elevations of these two horizontalsurfaces may or may not be identical. Options are discussed later inconnection with roller assembly design.

[0108] As a help in contrasting the first powder bed assembly 202 withthe second powder bed assembly 302, described later, it is helpful topoint out that in the first powder bed assembly 202 there is only oneperimeter wall 220, i.e., there is only one component against which therounded-rectangle gasketed outer surface of the carrier plate 222 everslides during its entire range of vertical motion.

[0109] Returning to FIG. 7, the illustrated powder bed piston assembly204 is positioned between the lower walls 212 of the first powder bedassembly 202 and moves vertically to engage with, disengage from andchange the vertical position of the carrier plate 222. The uppermostelement of the first powder bed piston assembly 204 is a vacuum coupling230 which can use a seal, such as an O-ring 232, to facilitate a vacuumseal between the vacuum coupling and the bottom surface 224 of thecarrier plate 222. The vacuum coupling is shown disengaged from carrierplate 222. The vacuum coupling 230 can be configured—such as with ribsor other features—to support carrier plate 222 at multiple points withinthe vacuum region and thereby avoid causing deformation or deflection ofcarrier plate 222 when carrier plate is engaged with the vacuum coupling230. The use of a carrier plate 222 with a flat bottom, together withvacuum coupling 230, means that it is easy to establish or breakmechanical connection to facilitate removal and replacement of the firstpowder bed assembly 202 or second powder bed assembly 302. It alsoprovides that establishment of the coupling is relatively insensitive tominor variations in the exact placement of the first powder bed assembly202 or second powder bed assembly 302 onto the three-dimensionalprinting machine 100.

[0110] The first powder bed piston assembly 204 moves with respect tothe first powder bed motion controller 206, which is mounted byfasteners 234 to the table-top 104. Thus, when the first powder bedpiston assembly 204 moves with respect to the first powder bed motioncontroller 206, the vacuum coupling 230 moves with respect to the firstpowder bed 202 and table-top 104. A bellows 236 surrounds a rod 238, andis configured to allow the rod to extend and retract with respect to thefirst powder bed motion controller 206. The bellows 236 can preventpowder which might pass between the carrier plate 222 and the perimeterwall 220 from traveling to and interfering with the first powder bedpiston assembly 204 or first powder bed motion controller 206.

[0111] The first powder bed motion controller 206 is mounted to theunderside of the table-top 104, and controllably moves the rod 238 toultimately change the position of the carrier plate 222 with respect tothe first powder bed assembly 202. A motor 240 may be coupled by a bevelgear 242 to the rod 238, and is controllable from a motor control unit244 to move the rod as desired. A vacuum line 246 provides a vacuum froma vacuum source 247 to the vacuum coupling 230, and a vacuum sensor 248is operable to confirm that the proper amount of vacuum is provided tothe vacuum coupling. The system can be configured to increase ordecrease the vacuum as necessary, or to notify the operator if thevacuum is outside of acceptable operating parameters. The inventorsappreciate that, after reviewing this disclosure, an individual of skillin the art will appreciate that there are other systems and methods forraising and lowering powder, which would be considered equivalent tothat described and illustrated herein. The first powder bed assembly 202is shown as being the feed bed assembly in three dimensional printingmachine 100, but it should be understood that first powder bed assemblycould be used as the build bed assembly instead of or in addition to itsuse as the feed bed assembly. If it is used as the feed bed, it can befilled with powder either in place on the machine or away from themachine in a separate location.

[0112] FIGS. 11-19 illustrate a more complicated embodiment of a secondpowder bed assembly and positioning system 300. This design of thesecond powder bed assembly 302 provides a tray subassembly that issmaller than the entire second powder bed assembly. Just as in thepreviously described first powder bed assembly, this second powder bedassembly 302 can be removed from the three-dimensional printing machine100 and replaced. In addition, in this design the tray subassembly canbe removed from the second powder bed assembly 302 and be replaced. Thepowder bed assembly and positioning system 300 is in large partequivalent to the first powder bed assembly and positioning system 200described above and illustrated in FIGS. 7-10. The elements of thesecond powder bed assembly and positioning system 300 that differ fromthe first powder bed assembly and positioning system 200 are discussedin detail below. It can be assumed that the details of the second powderbed assembly and positioning system 300 not discussed below may be thesame as or similar to those discussed above in connection with the firstpowder bed assembly and positioning system 200. Many of thesedifferences are equivalent and/or interchangeable, and could work justas well on the second powder bed assembly and positioning system 300 asthey do on the first powder bed assembly and positioning system 200.Accordingly, many of the following details can also be consideredalternate embodiments of those described above.

[0113] The second powder bed assembly and positioning system 300 isgenerally made up of a second powder bed assembly 302, a second powderbed piston assembly 304 and a second powder bed motion controller 306.The second powder bed assembly 302 rests on a pair of opposing globalguides 308. Similar to the first powder bed assembly 202, the secondpowder bed assembly 302 can be slid into and out of thethree-dimensional printing machine 100 when desired, and can beconfigured with position sensors and retainer members to confirm thatthe powder bed is in the proper operating position and stays in theoperating position. The second powder bed assembly 302 comprises a localguide 309 which engages with a tray subassembly 311. Tray subassembly311 comprises a perimeter wall 320 that surrounds carrier plate 322.

[0114] In the illustrated embodiment, the perimeter wall 320 of the traysubassembly 311 is much shorter than the perimeter wall 220 of the firstpowder bed assembly 202, in the vertical direction as viewed in FIG. 11.A carrier plate 322 closely conforms to the inner surface of perimeterwall 320, and is controlled by the second powder bed piston assembly 304to move the carrier plate 322 vertically with respect to the perimeterwall 320 and the rest of the second powder bed assembly 302.

[0115] As illustrated in FIGS. 16 and 17, a gasket 326 extends aroundthe perimeter of the carrier plate 322. The gasket 326 extends aroundthe perimeter of the carrier plate 322 within a groove 323, and meets atits opposing ends in a scarf joint 327. The scarf joint 327 may allowthe gasket 326 to move, expand and contract lengthwise, without thematerial of the gasket buckling. The illustrated gasket is made frompolymer in the case illustrated here involving a scarf joint, but ingeneral the gasket could instead be made of felt or any of the materialspreviously described in connection with the first powder bed 202 foreither powder bed assembly.

[0116] As illustrated in FIGS. 11, 18 and 19, the tray subassembly 311is held in the operating position by a release lever 329, which in theillustrated embodiment is a handle 331 that is operable to retain orrelease the tray subassembly 311 in the second powder bed assembly 302.A pin 333 extends through the release lever 329 at a point centrallylocated between the handle 331 and an operating arm 335 opposite thehandle. The pin 333 engages the adjacent structure in the second powderbed 302, and allows the release lever 329 to pivot with respect to thesecond powder bed 302.

[0117] When the handle is in the downward-most position, as illustratedin FIG. 18, the operating arm 335 of the release lever 329 is positionedabove the bottom edge of the tray subassembly 311. As a result, the traysubassembly 311 is locked in its operating position. However, when thehandle 331 is raised, the release lever 329 pivots about the pin 333,and the operating arm 335 rotates downward to a point where it is belowthe bottom edge of the tray subassembly 311. In this position, the traysubassembly 311 is free to be removed from the powder bed assembly 302,as illustrated in FIG. 19. The weight of the handle 331 causes therelease lever 329 to automatically return to the locked position whenthe operator lets go of the handle. The handle 331 can be configuredwith a sensor and/or alarm to register when the handle is locked orunlocked, and to notify the operator of the same.

[0118] The second powder bed piston assembly 304 and the second powderbed motion controller 306 of the second powder bed assembly 300 arestructurally and functionally very similar to those in the first powderbed assembly and positioning system 200. One exception, however, is thatthe second powder bed motion controller 306 can utilize a direct linkagebetween a motor 340 and a piston rod 338, as opposed to incorporatingthe bevel gear 242. In the case of the first powder bed 302, the carrierplate 322 typically moves only downward during operation of thethree-dimensional printing machine 100. Consequently, the force requiredto move the second powder bed assembly 302 is typically less than thatrequired to move the first powder bed assembly 202. Thus, the bevel gear242—which increases the force that the motor 240 exerts on the firstpowder bed piston 204—is not necessary in the illustrated second powderbed assembly and positioning system 300.

[0119] The tray subassembly 311 contains a carrier plate 322, and a traysubassembly perimeter wall 320 that closely fits around the carrierplate. There may also be a gasket 326 between the carrier plate 322 andthe tray subassembly perimeter wall 320. The tray subassembly 311 may beonly slightly larger in its horizontal dimensions than the carrier plateitself.

[0120] During use, as the carrier plate 322 executes its full range ofvertical motion, at lower elevations the carrier plate slides withrespect to the tray subassembly perimeter wall 320, and at higherelevations the carrier plate slides with respect to the feed bedassembly perimeter wall 321. The inside surfaces of these two perimeterwalls, against which the carrier plate slides, have closely matchingshape and dimension and may be accurately lined up with each other. Thealignment is achieved in part by local guides 309 within the powder bedassembly (which are different from the global guides 308, which controlthe placement of the powder bed assembly relative to the overallthree-dimensional printing machine 100).

[0121] The three-dimensional printing machine 100 may include sensors todetect both the presence of the overall powder bed assemblies and thepresence of the tray subassembly 311 within the second powder bedassembly 302, and possibly the position of the handle 329 as well. Thesesensors can communicate with an alarm or can act to shut off or disablecertain machine functions.

[0122] It is worth pointing out that although the illustrations show thefeed bed as being the simpler first powder bed assembly and positioningsystem 200 and the build bed as being the more complicated second powderbed assembly and positioning system 300 featuring the removable traysubassembly 311, either the feed bed or the build bed could be eitherdesign of powder bed assembly in any combination.

[0123] The second powder bed assembly 302 has all of the same overallfeatures as the first powder bed assembly 202 and it is similarlypossible to remove and replace the entire build bed assembly by slidingit out on the tracks to the side of the machine, such as for changingover from one powder to another or doing major cleaning of the machine.This would certainly be done occasionally. However, then there isanother capability of the second powder bed assembly and positioningsystem 300.

[0124] The first powder bed assembly 202 and/or the second powder bedassembly 302 may be removed and replaced after manufacturing one set ofparts and before manufacturing another set of parts. The easyreplaceability of the powder bed assemblies also permits easy changeoverfrom one materials system to another. It may be desirable to use thethree-dimensional printing machine 100 for more than one type of powder.For example, within the medical field it may be desirable to usedifferent families of powder materials for different bone substituteproducts, and for still other categories of medical products it may bedesirable to use still other powder materials. The use of removablepowder beds means that the feed bed (shown in the illustrated embodimentas 202) and the build bed (shown in the illustrated embodiment as 302)can be cleaned and/or filled with a different powder material outsidethe machine, where cleaning is easier. There may be powder bedsdedicated to particular materials, and the powder beds may simply beswapped out as part of a changeover of the machine from one powdermaterial to another.

[0125] If the first powder bed assembly 202 and the second powder bedassembly 302 are dedicated to individual powders, then changeover fromone powder to another is fairly quick (e.g., remove old beds, cleanloose powder from working areas of machine, such as by vacuum, wipeworking areas of machine especially datum plane, clean roller, and theninstall new beds).

[0126] The tray subassembly 311 would be removed and replaced far moreoften than the second powder bed assembly 302 as a whole. The traysubassembly 311 would be removed and replaced for each individual printjob, which could be more than once a day. In contrast, the powder bedassemblies as a whole might be removed and replaced only for changeoverfrom one powder to another or for major cleaning of the machine. It canbe appreciated that changeout of a tray subassembly is easier thanchangeout of an entire powder bed assembly. A further advantage ofchangeout of either a tray subassembly 311 or a powder bed assembly isthat for some circumstances of three-dimensional printing, it isadvisable to let the printed object remain in its build bed to finishdrying out for some period of time after completion of three dimensionalprinting, before the unprinted powder is removed. The replaceability ofthe tray subassembly 311 or the first powder bed assembly 202 allowsthis waiting and drying to occur without delaying the use of thethree-dimensional printing machine 100 for its next job. Furthermore, ifit is desired to perform drying at an elevated temperature before theunprinted powder is removed, the replaceability of the second powder bedassembly 302 or tray subassembly 311 allows this drying to be done awayfrom the three-dimensional printing machine 100 without impacting theoperation of the machine. Also, the replaceability of the first powderbed assembly 202 or its tray subassembly (if it has a tray subassembly)allows the supply of powder to be loaded (including tasks such assmoothing the powder) without using operating time of the machine.

[0127] As best illustrated in FIG. 2, a pair of opposing siderails 250extends along the three-dimensional printing machine 100 on opposingsides of the powder beds. The siderails 250 run in the direction of theslow axis. As shown, the siderails 250 are separated from the perimeterwalls 220 and 320 by channels or gaps 252 into which excess powder canfall, thereby minimizing the amount of powder reaching the siderails250. Each of the siderails 250 is positioned slightly outside of theperimeter walls 220 and 321. The siderails 250 in the illustratedembodiment can be flat on their top surfaces, so that complementarywheels on the heater assembly 600 and the roller assembly 400 can rollalong the siderails during operation (as discussed below). In addition(as also discussed below), the sliders 432 attached to the rollerassembly 400 also can travel along the tops of the perimeter walls 220and 321 to prevent powder which may be pushed out of the powder beds202, 302 by the roller 402 from reaching the siderails 250. Anindividual of ordinary skill in the art, having reviewed the presentdisclosure, will appreciate that there are many equivalent structuresthat can be substituted to perform these functions. In addition topowder control by structures such as sliders described later, it can beappreciated that during roller spreading it is possible for powderparticles to become airborne and thereby spread elsewhere in thethree-dimensional printing machine, which can eventually interfere withproper operation of various components. It is believed that rolling ofcomponents on particle-coated surfaces rather than on clean surfaces isa source of positional error and other problems in three-dimensionalprinting machines. Accordingly, it is also possible to create localizedairflow, which should help prevent airborne particles from migrating toother parts of the three-dimensional printing machine 100. This airflowcan be directed into the gap or channel 252. It can be created bysuction into gap or channel 252 or by an air knife (a jet which isnarrow in one direction and long in another direction) directed into gapor channel 252. Powder-laden air that is received therein would have tobe filtered or removed from the machine to complete the capture of thoseairborne powder particles.

[0128] For either design of powder bed assembly 202 or 302, the carrierplate 222, 322 may comprise recesses or wells in its top surface toserve as places for collection of slurry carrier liquid which percolatesdownward through the deposited powder if slurry deposition of powderlayers is used. With slurry deposition of powder layers, it would alsobe possible that the carrier plate 222, 322 be provided with holes orporosity for drainage of carrier liquid threrethrough. Such drainagecould be assisted by vacuum or suction provided either by a dedicatedvacuum or suction system, or by the vacuum system which is used for thevacuum gripping at the underside of the carrier plate.

[0129] Roller Assembly

[0130] FIGS. 20-23 illustrate a roller assembly 400 according to thisparticular embodiment of the present invention. The roller assembly 400is mounted to the first carriage 126, as illustrated in FIG. 20.Accordingly, the roller assembly 400 moves with the first carriage 126as the first carriage moves along the length of the three-dimensionalprinting system 100.

[0131] An elongated roller 402 is mounted in an elongated head assembly404, and is free to rotate about an axis of rotation 406 (FIG. 21) withrespect to the head assembly. The roller 402 can be controlled to rotatein either or both directions, or to remain in a fixed orientation, withrespect to the head assembly 404. If the roller 402 is controlled torotate in the direction of travel, the roller can either rotate at thesame speed as the surface it is rolling over (i.e. with no relativemovement therebetween), or can rotate faster or slower than the rate oftravel. In addition, the roller can be controlled to change its speedand/or direction of rotation during operation, as desired.

[0132] The roller 402 has an enlarged, cylindrical operating surface 408that extends along substantially the entire length of the illustratedroller. At opposing ends of the roller 402, a neck 410 (F IG. 21) ofreduced diameter engages an endplate 412 in the head assembly 404. Abearing or other feature may be positioned between the engagementsurfaces of the neck 410 and the endplate 412 to facilitate rotation ofthe roller 402 with respect to the endplate.

[0133] As best illustrated in FIGS. 2 and 4, the roller 402 is longenough to extend widthwise across the first and second powder beds 202,302, so that the roller can spread powder from the feed bed across thesurface of the build bed. There are at least two possibilities formaintaining the vertical elevation of the roller and thereby thethickness of the layer of spread powder. One possibility is that theroller 402 rolls on the siderails 250, maintaining a constant elevationwith respect to the tops of the first and second powder beds 202, 302,sometimes referred to as the datum plane 154 (FIG. 2). The top of thesiderails 250 can be level with the top of the second powder bed 302(top of perimeter wall 321), or can be spaced above the top of the buildbed to create a minimum layer thickness. If the roller rolls along thesiderails and the powder layer thickness is defined by the inequalitybetween the elevation of the siderails 250 and the tops of perimeterwalls 220 or 321, it is possible for the elevation of the siderails tobe approximately 0.005 inch (127 microns) higher than the elevation ofthe tops of perimeter walls 220 or 320. In this strategy, the rollerdoes not contact the top of perimeter wall 220 or 320 during powderspreading; instead it rolls on siderails 250 which are further from thepowder bed assembly and can be at their own vertical elevation. In thisinstance it may be useful to employ methods to limit the migration ofloose powder, such as sliders 432 and airflow patterns, to keep thesiderails 250 relatively free of powder. Brushes or similar features mayalso be used. When the roller 402 moves along the siderails 250spreading powder, the thickness of the powder layer can be controlled bythe height of the siderails 250 above the second powder bed 302, and bythe distance between the roller 402 and the siderails. As discussedbelow, the datum plane 154 may be used by several elements of thethree-dimensional printing machine 100 to determine the relative heightof the element with respect to the powder in the beds.

[0134] It is also possible that the height of the roller 402 is notdetermined by any rolling action on siderails 250 or other components inthe vicinity of the first and second powder beds 202, 302 but rather isdetermined to the position of the motion system relative to tracks 130on the gantry 106. In this strategy also the roller 402 does not contactthe top of the perimeter wall 220 or 320. A small gap such as 0.005 inch(127 microns) may be maintained between the roller 402 and the perimeterwall 220, 320 of the beds. In such a case a gap may also be maintainedbetween the roller 402 and the siderails 250. It is possible that such asystem provides more consistent powder layer thickness than a systemwhose vertical elevation is determined from a local surface that issubject to the presence of powder that may alter a sensed elevation. Itis also possible that the existence of a gap between the roller 402 andother solid surfaces is helpful for enabling the roller to be keptrotating at times when rolling is not being performed, which is helpfulwhen a roller heater is being used.

[0135] In such a system, it is possible that the roller assembly 400 inthe vicinity of the roller 402 may have a feature to permit the verticaladjustment of the elevation of the roller assembly. The roller assembly400 may be pivotably movable with respect to the carriage 126 or upperstructure that is connected to the rails, so as to cause adjustment ofthe vertical position of the roller 402. A possible design foraccomplishing this is that such adjustment feature have a stop in such away that the roller assembly 400 rests on the stop to determine itselevation by limiting its downward motion, but that the roller assemblybe unrestrained against upward motion (at least within a defined range).The exact effective position of the stop, for determining the roller 402location in its vertically downmost position resting against the stop,may be determined by shims. It may be useful to provide two such stops,one on each side of the beds. The ability for unrestrained upward motionwithin a defined range will allow the roller assembly 400 to be betteraccessed for cleaning and other maintenance operations. It can alsoprovide a degree of protection of parts against major damage in theevent of the roller 402 striking something during rolling.

[0136] The operating surface 408 of the roller 402 can be polishedchrome and the siderails 250 can be Teflon, so the roller is able tofreely slide or roll along the siderails without significant resistancedue to friction. The inventors appreciate that there are many equivalentstructures and materials for performing these functions, variations thatdo not deviate from the spirit of the present invention.

[0137] As best illustrated in FIG. 21, a first end 414 of the roller 402is engaged with the corresponding endplate 412 to rotate freely withrespect to the endplate. An opposing second end 416 of the roller 402 isconfigured to drive the roller's rotation. To that end, on the outsideof the endplate 412 on the second end 416 of the roller 402, a pulley418 is coupled between the neck 410 of the roller and a belt 420 that iscontrollable to rotate the roller. As best illustrated in FIG. 22, thebelt 420 is engaged not only with the pulley 418, but also with a drivepulley 422 and a tensioning pulley 424. The drive pulley 422 is coupledto a drive motor 426, which is in turn coupled to a controller 428 toallow the operator to selectively control the direction and rate ofrotation of the roller 402 with respect to the head assembly 404.

[0138] The tensioning pulley 424 is engaged with the endplate 412 at anelongated groove 430, and is adjustable along the length of theelongated groove to increase or decrease the tension in the belt 420, asgenerally understood in the relevant art.

[0139] Spaced apart from each other along the length of the headassembly 404 in the direction of the axis of rotation 406 are a pair ofsliders 432, which are mounted to a faceplate 434 on the head assembly.As illustrated in FIG. 21, an extreme lower edge 436 of the slider 432can align vertically with the bottom of the operating surface 408 of theroller 402, or with the top of perimeter wall 220, 321. The two sliders432 may prevent powder from escaping to the lateral sides as the powderis moved from the first powder bed 202 to the second powder bed 302. Theinventors appreciate that, depending on the particular system, the size,spacing and positioning of the sliders 432 could vary dramaticallywithout deviating from the spirit of the invention. For example, thesliders 432 can be coupled to the head assembly 404 in a manner thatallows the sliders to move up and down, and can be biased downward tomaintain contact between the sliders and the siderails 250. Similarly,the pair of sliders 432 could be replaced by more or fewer sliderswithout deviating from the spirit of the invention.

[0140] Printhead Assembly

[0141] FIGS. 24-27 illustrate a printhead assembly 500 according to thisparticular embodiment of the present invention. As best illustrated inFIG. 25, the printhead assembly 500 generally incorporates a supplyassembly 502, a plate assembly 504, a support frame assembly 506, anelectrical connector assembly 508, and an orifice plate assembly 510.The supply assembly 502 has a number of fittings 512 through whichfluids, such as binder, are introduced into the printhead assembly 500.In the illustrated embodiment, the supply assembly 502 has eightfittings 512, although this number can be varied without deviating fromthe spirit of the invention. The inventors also appreciate that theprinthead assembly 500 can incorporate manifolds in place of separatefluid flow channels, without deviating from the spirit of the invention.

[0142] In summary, as discussed in more detail below, the supplyassembly 502 provides fluid to the adjacent plate assembly 504, which isabutted against the supply assembly during use. Opposite the supplyassembly 502, the plate assembly 504 is connected by tubing 514 to anumber of solenoid valves 516, which are housed in the support frameassembly 506. During operation, the solenoid valves 516 dispense thefluid through the orifice plate assembly 510, toward the top surface ofthe second powder bed 302. A number of fin members 518 are mechanicallyand thermally coupled to the solenoid valves 516 and may help controlthe temperature of the solenoid valves.

[0143] FIGS. 29-31 further illustrate the supply assembly 502. The body522 in the illustrated embodiment is made from two separate parts, anupper portion 528 and a lower portion 530. An upper chamber 524 islocated within the upper portion 528 of the body 522, while acomplementary lower chamber 525 is located in the lower portion 530 ofthe body. Upper chamber 524 can be vented, to release possible gasbubbles, by screw vent 527 which may have a hollow center or milled slotcommunicating with the outside when the screw vent is not tightened, andmay have an O-ring to seal when the screw vent is tightened. Asillustrated in FIG. 30, each of the fittings 512 is adapted to connect asupply hose 520 to the upper portion 528 of the body 522 of the supplyassembly 502. Fitting 512 may incorporate an in-line filter. A port 526allows fluid from each supply hose 520 to pass through the body 522 andinto the respective upper chamber 524.

[0144] A gasket 532 is positioned between the upper and lower portions528, 530 of the body 522, and prevents fluid from passing from the upperchamber 524 into the lower chamber 525. The lower chambers 525 in theillustrated embodiment can be filled with air or another compressiblegas at or near atmospheric pressure. The gasket 532 is sufficientlyresilient to deform into and out of the lower chambers 525 in responseto pressure fluctuations in the fluid, which may stabilize system fluidpressure and reduce or eliminate the effects of water hammer.

[0145] An exit port 534 (FIG. 30) is positioned in the upper portion 528of the body 522. In the illustrated embodiment, each upper chamber 524has one entrance port 526 and one exit port 534, and the exit ports 534are positioned opposite the respective entrance ports 526. One ofordinary skill in the art, after reviewing this disclosure, willappreciate that equivalent configurations exist for performing thesefunctions, without deviating from the spirit of the invention.

[0146] The plate assembly 504 is best illustrated in FIG. 28. Theillustrated plate 536 is adapted with eight apertures 538, extendingacross the width of the. Each aperture 538 is positioned to align withthe corresponding exit port 534 in the supply assembly 502 when theplate 536 is abutted against the supply assembly. As a result, the fluidexiting each upper chamber 524 in the supply assembly 502 passes throughthe plate 536 at a corresponding aperture 538. Each aperture 538 in theillustrated embodiment is surrounded by an O-ring 540 to prevent thefluid from escaping. The removable plate assembly allows quick changeoutof microvalves for replacement or for cleaning them off-line. Theinventors appreciate that the number and placement of the apertures canvary to accommodate different supply assemblies, without deviating fromthe spirit of the invention.

[0147] As illustrated in FIG. 27, a length of tubing 514 extends fromeach aperture 538 in the plate 536 to a corresponding solenoid valve516. Each length of tubing 514 engages the plate 536 on the sideopposite the supply assembly 502.

[0148] FIGS. 32-34 illustrate a solenoid valve 516 according to thisparticular embodiment of the present invention. The illustrated solenoidvalve 516 can be purchased from Lee Company of Westbrook, Conn. Eachsolenoid valve 516 has an inlet 542 and an outlet 544. The inlet 542 isconnected by the tubing 514 to the plate 536 during operation, andreceives fluid from the supply assembly 502. The outlet 544 dispensesprecise amounts of fluid through the respective orifice tubes 545 andout of the corresponding orifice 547. Orifice tubes are connected tooutlet 544 by a sleeve 543. A dedicated orifice plate 549 (FIG. 38 inthe nozzle/orifice plate assembly 510 maintains each orifice tube 545and orifice 547 in the desired alignment. Because each orifice tube 545is retained in position by a dedicated orifice plate 549, each solenoidvalve 516 can be replaced independently, which may reduce down timeassociated with replacing and aligning the orifice tubes 545. What isillustrated in FIG. 39 is an orifice tube, having essentially constantcross-section for most of its length and an orifice at the end. However,it would be equally possible to use a nozzle, which may be more taperedexternally and perhaps internally as well. A nozzle could equally wellbe aligned by a similar nozzle/orifice plate. Either would be connectedby a sleeve 543 to the valve outlet 544.

[0149] As best seen in FIG. 34, the illustrated valve body 546 houses aplunger 548 that moves axially with respect to the valve body 546between a closed position in which a plunger face 550 contacts a valveseat 552, and an open position in which the plunger face is spaced apartfrom the valve seat to allow the fluid to pass through the outlet 544. Aphysical stop 551 can prevent the plunger 548 from moving beyond adesired distance in the open direction.

[0150] The plunger is biased by a spring 554 that urges the plunger face550 into the closed position against the valve seat 552. In theillustrated embodiment, the plunger face 550 is made from a resilient,seal material such as EPDM (ethylene propylene diene) rubber or Viton ora like material to improve the seal when the solenoid valve 516 is inthe closed position. One of ordinary skill in the art, having reviewedthis disclosure, will appreciate that equivalent structures exist.

[0151] The illustrated solenoid valve 516 is opened and closed throughelectromagnetic actuation. A coil of electrically insulated wires 556encircling the plunger 548 is alternatingly energized and grounded via apair of terminals 558 (FIG. 32) to alternatingly open and close thesolenoid valve 516, respectively. An inner wall (bobbin) 560 separatesthe coil of wires 556 from the fluid passing through the solenoid valve516, and separates the coil of wires from the material of the valve body546. The space between the coil of wires 556 and the valve body 546 maybe filled with a thermally conductive epoxy (e.g. Omega-Bond OB-101,having a thermal conductivity of 1.038 W/m-K) to conduct heat dissipatedat the coil of wires 556, through the valve body and to the fin member518 or a similar heat sink.

[0152] The valves 516 are mounted or attached to the body of theprinthead by attachment members, which can also be fins for dissipationof heat. FIGS. 35-37 illustrate the fin member (heat sink) 518 accordingto this particular embodiment of the present invention. Each fin member518 has a mounting surface 564 that can be shaped to conform to aportion of the solenoid valve 516. Opposite the contact surface 564,each fin member 518 has a number of individual fins 566 spaced apartlaterally across the entire width of the contact surface 564. In theillustrated embodiment, each fin member 518 has four fins 566. Theinventors appreciate that the number of fins, and the size and shape ofeach fin can vary without deviating from the spirit of the invention.

[0153] Each fin member 518 in the illustrated embodiment has a pair ofopposing apertures extending therethrough for mounting the fin memberagainst the support frame assembly 506. The fin member 518 is thusreadily removable and replaceable with another fin member to providevarying thermal characteristics to the print head 500 based on theparticular demands or desires of the operator. In addition, the finmembers 518 can be individually removed and replaced, which may furtherfacilitate the replacement of individual solenoid valves 516, which mayexpedite repair of the printhead assembly 500.

[0154] As indicated above, the individual solenoid valves 516 can beremoved and replaced independently without affecting the other valves inthe printhead assembly 500. The fin members 518 can be individuallyremoved and attached to a new solenoid valve 516; the individual orificeplates 549 can be removed and replaced, and a calibration plate or otherdevice can be used to align the sleeves 545 and nozzles 547 in thedesired alignments.

[0155] The printhead assembly 500 as shown contains a chamber 525 forthe suppression of pressure surges, for each valve 516 or supply hose520. Alternatively, chambers could be combined. An individual chamber525 may be dedicated to a particular fluid if more than one fluid isdispensed by different parts of the same printhead. Different valveswithin the printhead assembly 500 can similarly be dedicated toparticular fluids if more than one fluid is dispensed by different partsof the printhead. In particular, microvalves are capable of dispensingsuspensions, at least up to a modest solids content. This has beendescribed in commonly assigned U.S. patent application Ser. No.09/991,556 entitled “Printing or dispensing a suspension such asthree-dimensional printing of dosage forms,” which is herebyincorporated by reference in its entirety. Accordingly, microvalves canbe used either for dispensing binder liquid, which contains suspendedparticles, or for depositing a layer of powder particles by slurrydeposition. Accordingly, it is possible that a single printhead assembly500 could contain both dispensers for depositing powder particles byslurry deposition and dispensers for dispensing binder liquid onto thosepowder particles. One modification of the valves which can be useful foruse with suspensions is to provide a bypass or alternate exit path fromthe valve, as described in the referenced patent application. Withsuspensions, in order to prevent sedimentation, it is useful to keep thesuspension in motion, even when there is no dispensing of suspension.Accordingly, a microvalve with the bypass feature provides an openflowpath up to very close to the valve seat, a flowpath that is alwaysopen and available for flow for the purpose of discouragingsedimentation. The bypass flowpath exits the side of the valve near thevalve seat. The printhead may have a feature designed in to it toprovide access for that bypass flowpath. Holes such as those shown inthe lower portion of FIG. 30 can provide the access.

[0156] As alternative ways of dispensing suspension or slurry fordeposition of layers, it would be possible to use other types ofstructures, such as pinch valves or peristaltic pumps. It would bepossible for any such apparatus to turn on and off repeatedly such asturning off during the end region (turn-around region) of each rasterpass of a raster motion for slurry deposition of powder layers. Thismight be useful in the case of depositing expensive powder materialssuch as hydroxyapatite. Turning off the flow of suspension or slurrycould enable all the raster lines of slurry to be deposited in the samedirection of motion as opposed to depositing them in both directions ofraster motion. Such valving action, or even control of a stepper motordriving a peristaltic pump, could be coordinated with instructionsdriving other parts of the three dimensional printing machine. Apparatusfor slurry deposition of powder is disclosed in commonly assigned U.S.patent application Ser. No. 10/122,129, which is hereby incorporated byreference in its entirety.

[0157] The printhead assembly 500 is moved to the desired position forprinting, preferably by a computer-controlled motion control system. Thecomputer-controlled motion control system controls the printhead driver142 and the fast axis motor 144 to move the printhead assembly 500 intothe proper position for dispensing fluid. The printhead driver 142 movesthe second carriage 128—and with it the printhead assembly 500—back andforth along the slow axis. The fast axis motor 144 moves the printheadassembly 500 with respect to the second carriage 128 along the fastaxis. In the illustrated embodiment, the printhead assembly 500 rides onan air bearing 150 (FIG. 5) along a lower cross-member of the secondcarriage 128. The inventors appreciate that movement of the printheadassembly 500 can be performed by a wide variety of equivalent structuresand control systems without deviating from the spirit of the invention.

[0158] Precision Dispensing of Fluid

[0159] One particular application for three-dimensional printing is thefabrication of medical products, such as pharmaceuticals and bonesubstitutes. Unlike traditional three-dimensional printing, discussed inthe Description of the Related Art, when dispensing ActivePharmaceutical Ingredients into a dosage form, it is important toprovide a delivered drop volume that is accurately controlled,measurable and repeatable to a close tolerance. This precision ensuresthat the final product has the proper amount of Active PharmaceuticalIngredient or other critical compound, within an acceptable tolerance.

[0160] For some medical purposes, it is necessary to dispense fluidswith low viscosity and low surface tension (such as organic solvents)while maintaining extremely tight flowrate tolerances. Unfortunately, ithas traditionally been difficult to maintain close flowrate tolerancesfor low surface tension/low viscosity fluids.

[0161] During operation of the present invention, the solenoid valve 516can be opened and closed in response to a peak-and-hold drive signal.Instead of a pulse signal, which had been used in the past but which wasincapable of dispensing precise and/or repeatable quantities of fluid,the inventors have discovered that a peak-and-hold drive signal allowsthe solenoid valve 516 to reliably and accurately dispensepharmaceutical agents and other fluids that require precise volumecontrol.

[0162] The fluid pressure in the illustrated embodiment has beendecreased as compared to the prior art, from approximately 11 psi toapproximately 5 psi, which allows the solenoid valve 516 to be left openlonger. Instead of a 200 microsecond pulse at 40 volts, for example, thecurrent system allows the valve to be opened with a 250 microsecond peaksignal at under 25 volts, followed by a 500 microsecond hold signal atunder 5 volts. The inventors appreciate that these figures are merelyexamples, and that one of ordinary skill in the art, having reviewedthis disclosure, will appreciate that there are many equivalentpeak-and-hold signals that will serve the inventive purpose withoutdeviating from the spirit of the present invention.

[0163] In addition, the inventors have discovered that, by using thepeak-and-hold drive signal, the energy used to open and retain thesolenoid valve 516 open is less than the energy previously required toopen and shut the valve using the pulse signal. Using the above figures,the total wattage dissipated, at an actuation frequency of 800 Hz,decreased from approximately 2.5W to approximately 1.1W. Thus, thesolenoid valve 516 of the present invention is less prone tooverheating, and may thus require less effort, structure and controls toremain at the desired temperature range. By maintaining a consistenttemperature, the solenoid valve 516 will also dispense fluid moreconsistently, as the viscosity of fluid is affected by the temperatureof the fluid.

[0164] Still further, the inventors have developed a method of selectingsolenoid valves 516 to maximize the precision of the dispensed dropvolume. Each solenoid valve 516 is tested and matched with other valvessharing particular characteristics, prior to being installed into thethree-dimensional printing machine 100. Even valves that have beenidentified by their supplier as being identical are tested and separatedinto different groups. It has been discovered that testing done bymanufacturers of microvalves does not actually indicate that the valvesin a common group have sufficiently similar flow characteristics, whencompared on an exacting scale required by the present invention.

[0165] Before installing the solenoid valves 516 onto the printheadassembly 500 of the three-dimensional printing machine 100, each valveis first tested by measuring the flowrate of liquid through the valve asa function of pulse width (using the peak-and-hold waveform). Theaverage drop mass is plotted against the pulse width for each valve. Theflowrates are plotted for total pulse widths ranging from 500-800microseconds. For the illustrated three-dimensional printing machine100, this is the operable range for the solenoid valve 516. Within eachpulse width, the “peak” portion of the waveform is approximately 500microseconds, and the “hold” portion of the waveform ranges fromapproximately 0-300 microseconds.

[0166] The solenoid valves 516 are separated into groups, every solenoidvalve in a particular group being capable of dispensing fluid at thesame flowrate as the other solenoid valves in the group, at some pulsewidth within the operational range of the valve. For example, if a firstsolenoid valve 516 has a flowrate of 20 units at 550 microseconds and asecond valve has a flowrate of 20 units at 750 microseconds, the firstand second valves would fall into the same group, because 550microseconds and 750 microseconds are both within the operable range ofthe solenoid valve 516. FIG. 40 illustrates the calibration curves ofeight valves which have been grouped because of the similarity of theirindividual calibration curves.

[0167] The solenoid valves 516 are then matched with their respectiveflow rate calibration curves, and valves from a common group areinstalled onto one of the printhead assemblies 500 for thethree-dimensional printing machine 100. The actuating pulse wave foreach solenoid valve 516 is independently controlled based on itsrespective calibration curve. As a result, each of the solenoid valves516 in a particular printhead assembly 500 may be controlled to dispensevolumes which are substantially identical to the volumes dispensed bythe other valves in the printhead assembly, as every valve in aparticular printhead assembly is taken from the same group and isindependently calibrated and controlled. For example, if a printheadassembly 500 incorporated the two valves described in the hypotheticalabove and it was desired that each microvalve dispense 20 units of fluidvolume per pulse, the first valve would be opened using a 550microsecond pulse and the second valve would be opened using a 750microsecond pulse. By so grouping, calibrating and controlling each ofthe solenoid valves 516 in the printhead assembly 500, the entireprinthead assembly can be controlled to dispense identical volumesthrough every dispenser 547. This situation can allow thethree-dimensional printing machine 100 to effectively manufacturepharmaceutical and medical devices requiring precise quantities offluids. It has been found that if valves are matched as determined bydata taken for one particular fluid that is Newtonian or substantiallyNewtonian, then they usually are matched for any other fluid which alsois Newtonian or substantially Newtonian. The phrase substantiallyNewtonian is used because the range of fluids suitable for such acalibration and matching technique can include fluids which are slightlynon-Newtonian, in either the shear-thinning or shear-thickening sense.

[0168] It has been found that on the scale of the graph in FIG. 40,valves can be considered well-matched with each other if they areseparated by no more than about 5 to 7 micrograms/drop on the verticalaxis. With a group of valves which are so matched, it is possible todraw a horizontal line through the individual characteristic curves ofall valves in a set at a desired flow rate or drop size, and that linewill intersect the characteristic curves of all of the valves within theusable and established range of pulse widths. It has been found that ifvalves are separated by approximately 10 micrograms/drop or more on thescale in FIG. 40, it is not possible to draw a horizontal line throughall of the curves or obtain the same size of drop from each valve bypulsewidth adjustment.

[0169] Microvalves (or valves, used interchangeably) can be manufacturedin a variety of sizes. Size need not refer to overall externaldimensions of the valve body, but rather may be understood to refer moreparticularly to the dimension of certain internal features through whichthe liquid flows. As discussed above, the opening/closing action of avalve may be defined by the relative position of a plunger and a seat,with the seat being a surface against which the plunger contacts when itis desired for the valve to be closed. One of the two contactingsurfaces, such as the plunger surface, may be elastomeric material. Whenthe valve is open, fluid flows through an annular region defined by thegap between the plunger 550 and the seat 552, and that point may bedefined by a dimension which is the inside diameter of the seat 552.Downstream of the seat inside diameter may be an exit tube 544, whichmay be of circular cross-section having an inside diameter either thesame as or different from the seat inside diameter. At the end of theexit tube may further be a nozzle having a minimum open cross-sectionaldiameter, which is the nozzle orifice diameter. The seat inside diametermay be used to describe a “size” of a valve, with further descriptionbeing provided by the exit tube inside diameter.

[0170] Typically, if designs of valves are varied by varying theorifice/seat diameter, then other components or dimensions of the valvemay also vary somewhat in conjunction with that variation of the seatinside diameter. For example, variations may be imposed on the insidediameter of the exit tube downstream of the orifice. Variations couldalso be imposed on the length of the stroke of the valve, although thathas not been done here.

[0171] It has been found that certain sizes of valves and certainviscosity ranges of fluids go well together to provide fluid deliverywhich is consistent, in variables such as drop size and flowrate, overlong periods of time. For example, it is believed that if a fluid has arelatively large viscosity, it is appropriate to provide relativelylarge seat inside diameter, exit tube inside diameter, and dimensions ofsimilar internal passageways to allow appropriate amounts of flow topass through it during the time interval that the valve is open.Similarly, it is believed that for relatively low viscosity fluids, itis appropriate to use smaller dimensions of the seat inside diameter andpossibly related components, so that there is still a non-negligibleamount of pressure drop at the seat of the valve. In the case of lowviscosity fluids, it has been found that valves which are relativelylarge (in view of the viscosity) are susceptible to unexplainedvariations in delivered flowrate or drop size, over long periods ofoperation, while valves whose size is in better relation (i.e., smaller)in view of the fluid viscosity are much more consistent in theserespects.

[0172] Three different sizes or dimension combinations of valves havebeen used in the present work, in conjunction with different ranges offluid viscosity. In approximate terms, the valve size may be matchedwith the fluid viscosity according to the guidelines in the followingtable. In this table, seat inside diameter (or seat orifice diameter)refers to the diameter of the central opening of the valve seat 552.Exit tube inside diameter refers to the inside diameter of exit tube 544in FIG. 34. In FIG. 34 these two diameters are shown as being identical,but in general they could differ from each other. Seat inside Exit tubeSelected Valve Diameter inside diameter Viscosity Range Designation inch(micron) inch (micron) (CentiPoise) Small 0.010 (250) 0.010 (250) 0.5 to5 Medium 0.017 (250) 0.017 (250) 5 to 10 Large 0.017 (425) 0.050 (1250)10 to 18

[0173] It is further believed that especially good flowraterepeatability is achieved by sizing the microvalve as a function of theliquid viscosity according to the above teachings, when this is alsodone together with the use of spike-and-hold actuation.

[0174] Interlayer Dryer Heater Assembly

[0175] FIGS. 41-43 illustrate a particular interlayer dryer heaterassembly 600 according to an embodiment of the present invention. In thefigures, the housing panel in the foreground has been “removed” to showthe elements inside the heater assembly 600. The illustrated interlayerdryer heater assembly 600 incorporates twelve heater elements 602 spacedapart from each other and held within a heater housing 604. The heaterhousing 604 is sufficiently long and wide to span across the entirelength and width of the second powder bed 302. As discussed below, inthis particular embodiment, the entire interlayer dryer heater assembly600 may be positioned over the second powder bed 302 during operation toapply heat to a newly printed-upon layer of powder in the bed, and couldbe removed to allow other manufacturing steps to be performed. In otherembodiments, the interlayer dryer heater assembly 600 can have differentdimensions, and one of ordinary skill in the art will, after reviewingthis disclosure, appreciate that there are many equivalent structuresfor the interlayer dryer heater assembly.

[0176] In the illustrated embodiment, the heater housing 604 is slightlylonger than it is wide, and in the vertical dimension it issignificantly shorter than it is long or wide. Essentially, the housing604 is slightly longer than a heater element 602, slightly wider thantwelve tubular heaters, and slightly taller than the diameter of aheater. The illustrated housing 604 is configured to allow cooling airto flow through the housing and around some or all of the heaters 602,maintaining the interlayer dryer heating assembly 600 at or below adesired temperature. Removing heat from the interlayer dryer heatingassembly 600 and transporting it to a location outside the immediatevicinity of the three-dimensional printing machine 100 may facilitatethe control of the temperature of the surrounding areas of thethree-dimensional printing machine 100. If the three-dimensionalprinting machine 100 is encased in an enclosure system 700 (discussedbelow), actively removing heat from the interlayer dryer heater assembly600 may be even more desirable. The inventors appreciate that, fordifferent applications and/or for different heating elements, the sizeand shape of the housing 604 will likely change without deviating fromthe spirit of the invention.

[0177] The illustrated housing 604 is solid on all four edges 606 and onits top panel 608. The bottom 610, however, is either open or is coveredwith a transparent material, such as glass, to allow heat to radiatetoward the powder in the second powder bed 302. Housing 604 may have oneor more gas inlet connections and gas outlet connections to establish aflow of air or other gas for the purpose of removing heat other than byreleasing it to the nearby surroundings. The flow of gas may be throughthe interior regions of housing 604 that are not occupied by heatingelements 602, and/or it may be along the length of the annular regionbetween glass tube 614 and outer tube 618 (FIG. 45), or it may be both.In order to further help prevent this heat from being released in theimmediate surroundings, fluid conduits such as flexible hoses may carrythis gas flow, in particular the exiting gas flow which has becomeheated, to a more distant place where it may be discharged.

[0178] Casters 612 are mounted about the perimeter of the housing 604 toallow the interlayer dryer heater assembly 600 to move into and out ofposition over the second powder bed 302. As best illustrated in FIG. 2,the heater assembly 600 rolls along the siderails 250. As discussedabove, the siderails 250 are positioned on either side of the secondpowder bed 302 and the first powder bed 202 beyond gaps or channels 252or 352. The interlayer dryer heater assembly 600 can thus move on thecasters 612 in the direction of the slow axis along thethree-dimensional printing machine 100, over the first powder bed 202,the second powder bed 302, or beyond the beds, where it is notpositioned over either bed. The interlayer dryer heater assembly 600 canbe moved into this third position, if desired, when the second powderbed 302 and/or the heater assembly 600 is cooling.

[0179] As also illustrated in FIG. 2, the interlayer dryer heaterassembly 600 of this embodiment is connected to the roller assembly 400such that motion of the roller assembly 400 also causes the same motionof the interlayer dryer heater assembly. The interlayer dryer heaterassembly 600 thus moves with the roller assembly 400 and with the firstcarriage 126, and is driven by the roller driver 140. The inventorsappreciate that the interlayer dryer heating assembly 600 could travelon a separate structure and could be propelled by a separate carriage orother form of motive means. One of ordinary skill in the art havingreviewed this disclosure will recognize many equivalent structures andcontrols, such as suspending the interlayer drying heating assembly 600from a carriage, none of which deviate from the spirit of the presentinvention. Types of heaters other than those described herein couldsimilarly be used.

[0180]FIGS. 44 and 45 best illustrate a heater element 602 according tothis particular embodiment of the present invention. The illustratedheater element 602 is tubular-shaped, and may be a “quartz” heater rod,which are commonly found in heating devices available to the generalpublic. Quartz heater rods have a reasonably small thermal mass, sowarmup and cooldown times are not excessive. These so-called “quartz”heaters are really a glass tube 614 with a coiled resistance wire 622inside, such as those commercially available from Casso-SolarCorporation, Pomona, N.Y. A particularly useful variety is sold underthe name Gold Unitube.

[0181] Radiation from the heater element 602 can be further directedtoward the bed by a reflector 616 on the surfaces of the glass tube 614oriented away from the beds during operation. To form the reflector 616,a portion of the surface of the glass tube 614 is coated with amaterial, e.g. gold, which reflects infrared radiation to direct theradiation in a specific direction, which in this case would be towardthe beds . In FIG. 45, the thickness of the coating is greatlyexaggerated for sake of illustration.

[0182] In the illustrated embodiment, the portion of the glass tube 614coated by the reflector 616 has the same pattern along the entire lengthof the tube. For this particular application, a useful fraction of thecircumference of the glass tube 614 to be coated with reflector 616 isat least half (to direct radiation downward) and preferably aboutthree-quarters (270 degrees out of 360 degrees) so as to direct theradiation more primarily downward, meaning the radiation from anindividual tube disperses over a range of only 90 degrees.

[0183] In the illustrated embodiment, the heater element 602 is encasedin an outer tube 618, and air or other cooling gases may be directedthrough a channel 620 created between the glass tube 614 and the outertube 618. Sensing the temperature inside the heater element 602, such aswith a thermocouple 626 (FIG. 44), allows verification that a particularheater element 602 is on and can be used to control the electrical powerdelivered to the heater element to closely control the temperature ofthe heater element.

[0184] The illustrated heater elements 602 (of outside diameter 0.625inch) can be available with a heat output of up to 80 Watts per inch oflength of the tube, although for present purposes it is anticipated thatit may be operated at around 31 Watts per inch of length of tube. Theheater elements 602 may be spaced apart from each other at acenter-to-center distance of about 1.25 inch, meaning that the averagepower density dissipated by the heater is about 25 Watts/inch{circumflexover ( )}2. The distance between the second powder bed 302 and thenearest surface of heater element 602 may be 1.125 inch. The wavelengthselected for operation of the heater element 602 is suitable (consideredmedium wavelength in heater terminology) for absorption by water, whichis a common binder base fluid needing to be evaporated.

[0185] An infrared (“IR”) temperature sensing device 624 is attached tothe interlayer dryer heater assembly 600 and is oriented to determinethe temperature at the surface of the second powder bed 302 during orfollowing interlayer drying. The electromagnetic radiation reaching theIR temperature-sensing device 624 can be filtered to a desiredwavelength to reduce or eliminate infrared noise from surroundingstructures. The IR temperature-sensing device 624 can be configured toobtain, register and deliver temperature information to the system tocontrol the power and duration of interlayer drying.

[0186] Enclosure

[0187] FIGS. 46-50 illustrate an enclosure system 700 according to oneparticular embodiment of the present invention. The illustratedenclosure system is generally conformed to the three-dimensionalprinting machine 100 described above and illustrated in the precedingfigures. As viewed in FIG. 46, the left side of the enclosure system 700corresponds to the side of the three-dimensional printing machine 100where the roller assembly 400 resides when not in use. Likewise, theright side of the enclosure system 700 corresponds to the side where theprinthead assembly 500 resides when not in use.

[0188] The enclosure system 700 generally incorporates an enclosure 702and a ventilation system 720, which can be configured to filter and/orrecirculate the air inside the enclosure. The enclosure 702 isconfigured not only to enclose the three-dimensional printing machine100, but also to allow the operator to view the three-dimensionalprinting process and access the elements of the three-dimensionalprinting machine.

[0189] The enclosure 702 incorporates a number of vertical structuralmembers 706 and horizontal structural members 708 linked together tocreate a pattern of large openings 710 spaced about the perimeter of theenclosure system 700. Each opening 710 is covered by a window 712 thatallows an operator to see into the enclosure 702, but that can preventthe transfer of air or debris into the enclosure system 700, and canprevent the transfer of vapors or other chemicals out of the enclosuresystem.

[0190] The enclosure system 700 may be substantially airtight whenclosed. For example, seals may be provided around substantially theentire perimeters of windows 712 to minimize leakage of air therethroughdue to any pressure difference that might exist. Seals may be made of acompliant material such as a polymeric material, which is compatiblewith organic solvent vapors if such vapors are present.

[0191] As best illustrated in FIG. 48, the lower window 712 on the frontand rear of the enclosure 702 are configured to open and close toprovide access to the three-dimensional printing machine 100.Accordingly, each of such windows 712 are attached to a hinge 714, whichis pivotally coupled to one of the vertical or horizontal structuralmembers 706, 708. A spring/dampener 716 is also linked to the hinge 714and extends to a separate location on the enclosure 702. Thespring/dampener 716 can be configured to pivot at both of its ends withrespect to the enclosure 702 and the hinge 714, respectively.

[0192] The windows 712 are shown in FIG. 48 in solid lines in theirclosed orientations, and are shown in broken lines in their openorientations. As illustrated, when the window 712 is in the closedposition, the hinge 714 rotates to compress the spring/dampener 716.When the window 712 is opened, the hinge 714 rotates with the window,and the piston 716 extends downward to follow the attachment point onthe hinge. When the window 712 is fully opened, the piston 716 is fullyextended, and exists being compressed to keep the window in the openorientation. The handle 718 on the side of the window 712 opposite thehinge 714 assists the operator in opening and closing the window.Windows that slide or execute other types of motion are also possible.

[0193] As viewed in FIG. 46, the lower window on the left-hand side canbe opened as described above to allow the operator to access or removethe first powder bed 202 or to insert a new feed bed, or to work on theroller assembly 400 or heater assembly (not shown). Likewise, the lowerright-hand window 712 can be opened to access, remove or replace thesecond powder bed 302, or to work on the printhead assembly 500. Thewindow sizes and shapes can be made appropriate for those purposes. Theshape, configurations and structures of the windows 712 can varydramatically without deviating from the spirit of the present invention.

[0194] There can be provided latches on the windows 712, or snaps, orsimilar positive features for maintaining the windows in a closedposition. As will be described shortly, it may be desirable to operatethe enclosure 702 at a pressure differential in either direction withrespect to the outside atmosphere. In at least one direction of pressuredifferential, latches may be useful for keeping the windows 712 closed.

[0195] The enclosure 702 may contain window closure sensors, located onor near windows 712, which detect whether a window is in a definedposition with respect to its adjacent frame thereby indicating closureof the window. Information from window closure sensors can be used toeither enable or disable one or more functions of the machine dependingon the position of the window 712 (open or closed). This can beaccomplished by integrating signals from window closure sensors withfunctions of the machine, using either hardwired controls or software.It is possible to implement such controls or software so that closure ofall windows 712 is necessary before the machine can begin operation. Itis possible to implement such controls or software so that the openingof a window 712 during operation of the machine will immediately haltone or more functions of the machine. Alternatively, it is possible toimplement such controls or software so that the opening of a window 712during operation of the machine will initiate a shutdown sequence, whichmay shut down one or more functions of the machine by means of aprogrammed sequence of events.

[0196] The enclosure 702 may include a ventilation system 720, whichcirculates and cleans the air within the enclosure system 700, andreplaces the air with fresh air from outside the system. The ventilationsystem 720, for example as illustrated in FIG. 49, can incorporate oneor more inlets 722 which allow external air to enter the ventilationsystem through a HEPA (High Efficiency Particulate Arresting) filter724. The inlet 722 can be connected by a hose 726 to an intake manifold728 that routes air from various inlets 722 to a blower 730. A blowermotor 732 powers the blower 730, causing air from the intake manifold728 to be forced through a heat exchanger or chiller 736. Clean air fromthe heat exchanger 736 can be routed by another hose 726 to an outlet738 that routes clean air to the enclosure 702. Outlet 738 can include adistribution plenum which spreads the airflow somewhat uniformly over asubstantial area, and a HEPA filter.

[0197] A recirculation port 740 may be used to draw air from theenclosure 702, through yet another hose 726 to the intake manifold 728.A pair of valves 742 can be manipulated to adjust the ratio of fresh airentering the ventilation system 720 from the external environmentthrough the inlet 722, to recycled air entering the ventilation systemfrom the enclosure 702 through the recirculation port 740.

[0198] In the described system, air flows in a generally horizontaldirection from one end of the enclosure 702 to the other. The air flowcomes into the enclosure 702 at the first powder bed 202 end of thethree-dimensional printing machine 100, flows toward the second powderbed 302 end of the machine, then exits the enclosure.

[0199] Clean rooms in various manufacturing processes are described asClass 100,000 or Class 10,000 or other classifications based on asystem, where the number indicates how many particles per volume of airare allowed. Class 100,000 is the crudest degree of cleanliness usefulfor medical purposes and is appropriate for manufacturing of medicalproducts, which are topical in nature (i.e., for application to theoutside the body). Class 100 cleanliness is typical for products thatmust be sterile. The degree of cleanliness that is achieved isdetermined in large part by the filter installed in the system, and alsoby parameters such as how clean the system was to begin with. In thepresent invention, the filters 724, and in outlet 738 discussed abovemay be selected so that the ventilation provides, in the interior ofenclosure 702, an environment having a cleanliness of the appropriatelevel desired for manufacture of a particular product.

[0200] It is possible to use some extent of recirculation of the airthat flows through the enclosure 702. Once-through air flow, i.e., norecirculation of air at all, may be undesirable in the sense that itinvolves filtering more air (with associated filtration costs) thanwould be necessary if some recirculation were used. However, completerecirculation of air may also be undesirable in the sense that it mightallow buildup in the air of undesirable vapors or other substances, iffor some reason the filters might not be effective in removing aparticular substance. For this purpose some degree of replacement ofairflow can help to remove and prevent buildup of any undesirablesubstances that may not be removed by the filter. A suitable degree ofrecirculation is to recirculate approximately 80% of the airflow duringeach passage of air through the system. This means that at the end ofpassage through the system, 20% of the airflow is diverted to exhaustand 80% of the airflow is passed into a return duct to go back into thesystem. This recirculated flow is supplemented by a new airflow equal inamount to the airflow that was extracted and sent to the exhaust. Thiscan be accomplished by valves having appropriate cross-sectional areaand other features.

[0201] The enclosure 702 around the three-dimensional printing machine100 can serve to prevent accidental entry of foreign objects oroperators' body parts into operating regions of the machine. Inaddition, the enclosure 702 and its ventilation system 720 can serve anyone or more of the following functions: maintaining a level ofcleanliness in the operating region of the machine; controlling theconcentration of vapors of binder liquid such as organic solvents;removal of heat that may be dissipated by various components in theoperating region, and associated control of the temperature of theoperating region; and even isolating the working region from random airmotions in the surrounding room. Furthermore, certain sensors that maybe associated with the enclosure may be used to influence operation ofthe machine.

[0202] The enclosure 702 may be operated at a pressure that is differentfrom the surrounding atmospheric pressure. The pressure inside theenclosure 702 may be a slight positive pressure, which is especiallysuited to preventing bacteria and other forms of contaminants fromentering the enclosure and the enclosed workspace. Alternatively, theenclosure 702 may be operated at a slight negative pressure, which isespecially suited to preventing vapors such as solvent vapors fromescaping from the enclosure. In achieving either goal, good seal designis also useful. Achieving either a positive or a negative operatingpressure inside the enclosure may be aided by a supplemental blower ateither the exhaust or the fresh air intake or both or connected directlybetween the enclosure interior and the exterior. Alternatively,appropriate sizing of flow areas in various parts of the flow system mayachieve the desired pressure situation without the use of componentssuch as supplemental blowers. The enclosure 702 may be maintained at adesired sub-atmospheric or above-atmospheric pressure even atessentially no flowrate of air through it, or at any desired flowrateranging from zero to full designed flowrate.

[0203] The illustrated enclosure 702 provides a number of advantagesover systems of the prior art. For example, the enclosure 702 may notonly provide clear visibility through the windows 712, but may alsoprevent debris or contaminants from entering into or escaping from theenclosure. The enclosure system 700 may also isolate the printing systemfrom the surrounding environment, which not only keeps thethree-dimensional printing machine 100 clean, but may also minimize oreliminate any random air movement within the enclosure system due tooutside influences, which could adversely affect the control of powderand/or droplets during operation. Still further, the enclosure system700 may help to maintain the proper internal temperature, which can becritical to fluid viscosity and other variables affecting performance ofthe system. Still further, the ventilation system 720 can be configuredto create a slight positive pressure on the inside of the enclosure 702,which may prevent contaminants from entering the windows 712 or otheropenings, due to differential pressures.

[0204]FIG. 50 illustrates an alternate ventilation system, incorporatingan auxiliary input cart 752 and an auxiliary output cart 754. Thealternate ventilation system 750 is designed to provide the samebenefits as the ventilation system 720 discussed above, but is modularin structure.

[0205] The auxiliary input cart 752 is configured with a blower forproviding air into the enclosure 702, and can incorporate a heatexchanger or other elements for providing the enclosure with air at adesired temperature and/or moisture content.

[0206] The auxiliary output cart 754 is configured with filters andadsorbers for removing undesired substances from the air in theenclosure, and with ducting and valving to allow the operator to controlthe ratio of new air to recirculated air in the enclosure.

[0207] The airflow through the enclosure 702 can be used to remove heatand thereby better control the temperature of operating componentsinside the enclosure. For example, heat may be dissipated by theinterlayer dryer apparatus 600 if that is part of the machine. Inaddition, heat can also be dissipated by a roller heater assembly 800 ifthat is used, and by other motors and electrical components, and by theblower itself. If no recirculation is used, or if the heat load isfairly small, it may be unnecessary to include an active chiller or heatexchanger component. However, in other cases it may be quite necessaryto include active components for control of the temperature of theairflow. Heat can be removed by passing the airflow through a heatexchanger, which may be mounted in the fluid flowpath downstream of theblower but upstream of the filter, before the air enters the enclosure702. Alternatively, the heat exchanger may be mounted in other places inthe flowpath. The heat exchanger may be a liquid-to-air heat exchangerreceiving chilled liquid from a chiller, which may be located remotefrom the other equipment.

[0208] It is possible that concentration of the vapor in the gas may becontrolled simply by the recirculation fraction and release exhaust ofthe diverted air into the atmosphere. This may be acceptable forsolvents which are relatively inexpensive and non-hazardous, such asethanol. Alternatively, air that leaves the working region may betreated by adsorption filtration or other means to remove vapors oforganic solvents and other possibly undesirable substances adsorptioncan bind the solvents in a solid form, and the adsorber can eventuallybe discarded. Alternatively, the vapors may be removed by a condensationoperation, even possibly permitting the re-use of those substances.Either type of treatment offers the advantage of removing the vaporsfrom the enclosure while at the same time preventing their release intothe atmosphere.

[0209] The air or gas handling apparatus of the present invention canalso include one or more sensors or devices for measuring theconcentration of organic solvent vapors in the air being handled. Theinformation obtained from such a sensor or device can be used togenerate a warning or alarm, or can be used as feedback to adjust anoperating parameter such as airflow, or can be used to cause a shutdownof at least some portion of the machine.

[0210] The present enclosure 702 also offers the possibility ofoperating the working region of a three-dimensional printing machine 100in an oxygen-deficient or inert atmosphere, should this be desired suchas for safety in dealing with combustible vapors. The ventilation 700system would have to be capable of supplying appropriate gascomposition.

[0211] The machine may also comprise facilities to supply a gaseous orvapor sterilant into the enclosure and to remove it. Such gases orvapors can include hydrogen peroxide vapor, ethylene oxide, and othersas are known in the art. For sterilization or disinfection of themachine, the sterilant gas or vapor may be supplied to the inside of theenclosure for an appropriate period of time and then may be removed.Materials of construction of those parts of the machine that are to beexposed to sterilant may be chosen to be compatible with the sterilant.

[0212] Instead of the equipment configuration illustrated, in which theair handling equipment is cart-mounted separate from the mainthree-dimensional printing machine 100, it might be possible to mountsome or all of that equipment underneath the three-dimensional printingmachine 100, depending on space availability.

[0213] In order to enhance cleanability, the enclosure 702 may bedesigned so that its interior surface contains a substantial number ofrounded internal corners, and so that the enclosure may be assembledusing a relatively small number of joints.

[0214] Interlayer Drying

[0215] As a general review and summary, the three-dimensional printingmachine 100 uses a first powder bed 202 that, for creation of a newpowder layer, rises slightly to present powder for each new layer to theroller 402. The roller 402 then moves across the first powder bed 202from left to right (in the orientation shown in FIG. 3) to spread newpowder onto second powder bed 302. The sliders 432 on either side of theroller assembly 400 prevent the powder from escaping from the side ofthe first or second bed 202, 302 as it is being spread.

[0216] After the new layer of powder has been spread, the printheadassembly 500 moves in a predetermined pattern over the second powder bed302 to dispense fluid in appropriate places to manufacture a layer ofthe desired product. Sequentially, the three-dimensional printingmachine 100 will by this method fabricate an entire product from thebottom up.

[0217] The printhead assembly 500 can be of the type illustrated inFIGS. 24-39A and discussed above, or any of various dispenser orprinthead varieties known in the art, including microvalve in eitherdrop-on-demand or line-segment mode, piezoelectric drop-on-demand,thermal drop-on-demand, continuous jet with deflection, etc.

[0218] In the illustrated embodiment, heater assembly 600 can be movedinto position above the second powder bed 302 and can direct heat outputtoward the second powder bed. As shown, heater assembly 600 isapproximately the same size as the entire second powder bed 302, i.e.,heater assembly 600 could move into position over the second bed, remainstationary, and radiate heat to the entire bed for an appropriate timeduration. The heater assembly 600 can then turn off and leave theoperating position to make room for further steps in thethree-dimensional printing operation. In the illustrated embodiment, theprinthead assembly 500 would have to move away from the second powderbed 302 to make room for the heater assembly 600 during the heating anddrying operation.

[0219] A particularly advantageous method of providing heat externallyis believed to be for heater assembly 600 to radiate heat, i.e., todeliver a significant portion of its heat as electromagnetic radiationin the infrared region of the electromagnetic spectrum. The heat can bedirected at its target fairly precisely and it may be at an appropriatewavelength to be absorbed by a liquid. This electromagnetic radiation isdirected at the second powder bed 302 upon which fluid has just beenprinted.

[0220] Alternatively, a plurality of lamps that emit in the infraredregion could also be used. As yet another alternative, a convectiveheater assembly could provide heat by producing a gentle flow of warmgas such as warm air, or a combination of radiant and convective heatingcould be used.

[0221] It would be possible for the heater assembly 600 to operate for apredetermined period of time in the expectation that it would dry theprinted region to the desired extent. However, it also would be possibleand would probably be preferable to have the operation of the heaterassembly 600 controlled by a control system, because of variousadvantages. For example, if one compares a layer containing a largeamount of printed binder with a layer containing a small amount ofprinted binder, there may be a larger amount of heat required for theformer than for the latter in order to reach the desired amount ofresidual binder for the start of the next pass of powder spreading ordeposition. A programmed heating cycle of fixed duration and dryingconditions could not distinguish between these two cases, but a controlsystem could distinguish between them and respond appropriately toachieve, for example, a desired bed temperature or liquid content orsimilar condition for the start of the next layer powder spreading orpowder deposition.

[0222] The control system could be responsive to a measured temperature,such as a measured temperature of the surface of the powder bed. Such atemperature measurement could be obtained by a non-contact temperaturemeasurement technique such as an infrared thermometer (currentlyavailable from Raytek, Santa Cruz, Calif.), which aims at apredetermined location on the bed. The spot at which it aims could be aspot that is part of the printed article, or it could be a spot that isprinted in a predetermined location simply for the purpose of having itstemperature measured. In one particular embodiment, it has beendetermined that optically filtering the thermometer to preferentiallypass a wavelength of approximately 7.9 microns may reduce the adverseaffect of radiation emanating from surrounding structural members.

[0223] Alternatively, the control system could be responsive to ameasured moisture content of a location in the bed. Moisture content canbe measured by a near infrared moisture sensor (currently available fromUnity Scientific of Beltsville, Md.). In one particular embodiment,immediately following printing, the moisture measured at a location thathas been printed is approximately 11%. It has been determined that amoisture content of approximately 8% after drying and before thesubsequent powder spreading and printing step may be desirable for goodlayer-to-layer binding. Moisture content may be considered to be thedifference between the weight of a sample and the weight the samplewould have if it were completely dry, normalized by the actual weight ofthe sample.

[0224] In the case of a non-contact temperature or moisture-measuringsensor such as an infrared detector for either temperature or moisturecontent, there are several possibilities for mounting it. Onepossibility is to mount an infrared sensor 624 (FIGS. 2 and 42) on theinterlayer dryer heater assembly 600, which comes into position over thesecond powder bed 302 only during the heating step. The otherpossibility is to mount the sensor in a fixed position on the frame ofthe three-dimensional printing machine 100. In this case the detectoralways points at a specified place on the second powder bed 302, but itwould have to be located somewhat further from the bed in order to allowthe printhead assembly 500 and its motion control apparatus to passunder it during printing.

[0225] In either case, it would be desirable to define a viewing pathfor the infrared radiation that passes between heaters 602 or otherelements of the heater assembly 600. The viewing path would preferablybe enclosed or shielded by a structure such as a sleeve that preventsradiation directly from the heaters 602 from reaching the sensor, andallows the sensor to view only its intended target.

[0226] Alternatively, the viewing line of sight for infrared measurementof temperature could avoid the heater assembly 600 by aiming at a cornerof the powder bed from angle so as not to intersect the heater assembly.In this case, where the sensor is closer to the second powder bed 302,cleaning apparatus could be provided to prevent the sensor fromcollecting powder. What is illustrated in FIG. 42 is a viewing path thataims at an angle so as not to intersect the heater assembly 600, withthe sensor 624 traveling with the heater assembly 600.

[0227] As yet another alternative, it would be possible to use fiberoptics capable of transmitting infrared radiation, which indicates thesurface temperature of the second powder bed 302. At a powder surfacetemperature of perhaps 80° C., in the case of water-based binderliquids, it might be suitable to conclude heating and move on to thenext manufacturing step of spreading the next layer of powder.

[0228] It would also be possible, of course, to use a thermocouple orsimilar device that contacts the second powder bed 302, but anoncontacting device has operational convenience and also enables atemperature reading to be taken which averages over a defined area.Alternatively, the heater assembly 600 could be controlled by a sensorresponsive to the moisture content (content of binder liquid) in thebed. Control decisions could be performed using a PID(proportional-integral-derivative) controller or algorithm as is knownin the controls industry.

[0229] It is possible that after repeated printing and heating oflayers, the second powder bed 302 will reach an asymptotic above-ambienttemperature and remain warm even as the printing is occurring. This mayhave some beneficial effect as far as accelerating evaporation of thebinder liquid, at least in the case of roller-spread dry layers. Ofcourse, this effect would not so much occur in the case of slurrydeposition of layers, because the liquid making up the slurry willprobably substantially cool off any heat remaining from the inter-layerdrying of previous layers.

[0230] In one variation, a small heater assembly (not shown) can besmaller than the length of the second powder bed 302, measured in thedirection of travel. For example, the small heater assembly mightconsist of only a single quartz rod heater element 602. This means thatthe small heater assembly would have to travel across the second powderbed 302 as it is drying, i.e., the heater assembly would simultaneouslymove and emit heat/radiation. In contrast to the previous example,drying would not occur simultaneously all over the second powder bed302. Rather, drying would occur in a band that advances across thesecond powder bed 302 as the small heater assembly advances.

[0231] One advantage of this latter variation would be the reduced spacerequired for the heater assembly, such as when it is out of use. If asufficient separation distance is provided between the printheadassembly 500 and the small heater assembly, it would even be possiblefor the small heater assembly to be heating a portion of the secondpowder bed 302 while the printhead assembly is printing on anotherportion of the build bed. This would reduce the total manufacturing timeneeded for all the steps associated with a layer. This would alsoprovide that each portion of the second powder bed would haveapproximately the same time interval between printing and application ofheat for drying, thereby reducing the possible bleeding in early-printedportions of a layer.

[0232] The following sequence illustrates one possible sequence of stepsinvolved in using interlayer drying with printing on a layer of powderthat has been roller-spread. In this case, the sequence of steps for twocomplete cycles of operation would be: Deposit a layer of powder ontothe second powder bed by roller- spreading; Print binder onto powderlayer; Perform inter-layer drying with external heat; Deposit anotherlayer of powder onto the second powder bed by roller-spreading; Printbinder onto powder layer; and Perform inter-layer drying with externalheat. It would also be possible to use interlayer drying on a layer ofpowder that has been slurry-deposited. In this case, the sequence ofsteps for two complete cycles of operation might be: Deposit a layer ofpowder onto the second powder bed by rastering a jet of slurry; Dryslurry layer, by external heat if desired; Print binder; Performinter-layer drying with external heat; Deposit another layer of powderonto the build bed by rastering a jet of slurry; Dry slurry layer, byexternal heat if desired; Print-binder; and Perform inter-layer dryingwith external heat.

[0233] The drying of the slurry carrier liquid could be performed by thesame apparatus as used for interlayer drying, or it could be performedby other means.

[0234] The present invention can be used with any method of generationof external heat including radiation, convection and combinationsthereof.

[0235] Optimum drying time and heating rate can be expected to depend onthe individual binder and perhaps also on the powder characteristics.Thus, they must be determined experimentally on a case-by-case basis.Whatever they turn out to be, the use of an external heater as in thepresent invention provides a way of adjusting the evaporation rate toachieve desired drying rate and extent.

[0236] There are also applications in which three-dimensional printingis used to produce oral dosage forms containing active pharmaceuticalingredients, for which the active pharmaceutical ingredients arecontained in the dispensed binder liquid. Frequently the solubility ofthese ingredients in binder liquids is limited, and the ability todeposit a desired dose into the selected pill size is an issue. Theability to deposit sufficient active pharmaceutical ingredient into thepill would be enhanced if multiple printing passes into the same powderlayer could be made. In such a situation, especially when the outside ofthe pill is an impermeable surrounding enclosure, it may not benecessary for the interior powder to actually be bound together into amonolithic solid. Rather, the interior powder could remain as loosepowder onto which is dispensed liquid that contains drug. In this case,by convention, the dispensed liquid might still be termed a binderliquid even though binding action is not occurring. It is entirelypossible that drug-containing liquid be dispensed onto the interiorpowder, the volatile liquid evaporates, more drug-containing liquid bedispensed onto the same powder, the volatile liquid again evaporates,and so on. In ordinary single-pass printing situations, the amount ofdrug that can be dispensed is limited by the solubility of the drug inthe liquid and the amount of liquid that can be dispensed in one pass toachieve the desired saturation level in the local powder. However, theuse of multiple passes provides the ability to arbitrarily increase theamount of drug deposited. An important part of this is that the powderdry sufficiently between passes so that more drug-containing liquid canbe deposited. Interlayer drying can help achieve this. When sufficientdrug has been deposited in a given powder layer, it may be desirable toactually bind the powder particles together. This would have to be doneusing a different liquid, which is actually a binder with respect to thepowder particles.

[0237] For this operation, using for sake of example double-passprinting, the sequence of steps would be: Deposit a layer of powder ontothe second powder bed by roller- spreading; Print binder onto powderlayer; Perform inter-layer drying with external heat; Print binder ontopowder layer; Perform inter-layer drying with external heat; Depositanother layer of powder onto the second powder bed by roller-spreading;Print binder onto powder layer; and Perform inter-layer drying withexternal heat. Print binder onto powder layer; Perform inter-layerdrying with external heat Of course, more than two passes couldsimilarily be performed.

[0238] The use of external heating to accelerate evaporation of thevolatile liquid in the binder, presumably between multiple printingpasses on the same layer, would make this a more attractive option. Infact, the active pharmaceutical ingredients may be contained in adispensed liquid that is, strictly speaking, not even a binder withrespect to the powder onto which it is dispensed. The dispensed liquidmay be a solvent for the active pharmaceutical ingredients but might notbe either a solvent for the powder or a carrier of an adhesive.Nevertheless, the dispensed liquid could serve the useful purpose ofbringing dissolved active pharmaceutical ingredients into the powder andthen evaporating, possibly with the aid of interlayer drying andpossibly even multiple times for a single layer of powder. This givesthe designer more options, for example allows use of a binder liquidwhich by itself is not sufficiently volatile to achieve a practicallyuseful evaporation rate at room temperature. For example, bromoform ischemically similar to chloroform (a solvent which is useful for manypurposes) and has the advantage of a noticeably higher surface tensionmaking dispensing easier, but has a poor evaporation rate. Interlayerdrying could help make the evaporation rate of a compound such asbromoform fast enough to be useful.

[0239] Heating the Roller

[0240]FIGS. 51 and 52 schematically illustrate a roller heater system800 according to still another embodiment of the present invention.After a layer of powder has been spread onto the second powder bed 302,the layer of powder may be printed upon by the printhead assembly 500dispensing one or more binder liquids, at least part of which is asolvent. The motion of printhead assembly 500 may be provided by aprinthead motion control system, such as that described above, wherein amotion control system comprises the second carriage 128 that moves alongthe slow axis, and a the printhead assembly 500 that moves along thefast axis. The solvent in the dispensed binder liquid may be allowed toevaporate to a desired extent, including if necessary application ofheat as interlayer drying. An interlayer drying apparatus, such as theheater assembly 600, may be positioned over the second powder bed 302for an appropriate time and operated at an appropriate power level tocause the desired extent of evaporation of solvent. Then, the entireprocess may be repeated.

[0241] Motion of the various components (i.e., the printhead assembly500, the heater assembly 600, the roller 402 and a roller heater 802)may be coordinated so that each is out of the way of the others at timeswhen any particular component needs to be above either the first powderbed 202 or the second powder bed 302.

[0242] The roller 402 may be heated by the roller heater 802, which maybe electrically operated. The roller heater 802 may be external to theroller 402, and one suitable mode of heat transfer is radiant heating,in which case roller heater 802 does not actually have to touch roller402. One suitable geometry for the roller heater 802 is an elongatedradiant heater. For example, quartz tube or other heaters may be made byenclosing an electrically resistive wire, possibly coiled, inside a tubesuch as a glass tube. As with the heaters 602 discussed above, someportions of the tube's surface may be left transparent to infraredradiation, while some other places may be coated with a reflectivecoating so as to direct infrared radiation in a desired direction. InFIG. 52, reflective coating 616 is shown as being located on portions ofthe surface of tubular roller heater 802 which are not facing the roller402, so as to direct thermal radiation in the general direction of theroller 402. Such a heater has relatively small thermal mass and so isrelatively quick to respond to changes in the power supplied to it.

[0243] For typical situations, a suitable heat output of the rollerheater 802 is 50 Watts/inch of length of the tube. The length of thetube may be approximately equal to the length of the roller 402 or maybe slightly shorter. With the use of the described tubular heaters 602,heat is transferred to the roller at least partly by radiation. However,it is also possible for at least some heat to be transferred to theroller 402 by convection. It is, for example, possible to supply heatedgas to the vicinity of the roller 402. Typically, heat will also be lostfrom the roller 402, such as by conduction to the mounting of the rollerand perhaps by other heat flow paths. It is possible to provide a shield804 or partial enclosure around parts of the roller heater 802 and theroller 402 to partially contain either heated air or thermal radiationand thereby reduce heat loss. The roller heater 802, and for that matterthe shield 804, could either move with the roller 402 when the roller402 moves during powder spreading, or one or both could be stationarywith respect to the machine chassis such that the roller 402 would onlybe in proximity to the roller heater 802 when the roller is in a restposition such that it is not involved in powder spreading.

[0244] It is possible that roller heater 802 could be powered during theentire three-dimensional printing process. Alternatively, it is possiblethat roller heater 802 may be powered only for a time period just priorto when the roller is scheduled to spread powder. Operation of rollerheater 802 may simply involve supplying it with a predetermined constantpower level. Alternatively, operation of roller heater 802 may begoverned by a control system (not shown) which uses a temperature sensor806 to measure the temperature of a place on roller 402 and whichgoverns the power level of the roller heater 802 in response to thatmeasured temperature. The temperature sensor 806 may be a non-contacttemperature sensor such as an infrared sensor, or may be a contacttemperature sensor, a thermocouple, a thermistor, a resistancetemperature detector, etc., as is known in the art. Alternatively,operation of roller heater 802 may be governed by a control system (notshown) which uses a temperature sensor (not shown) to measure thetemperature of roller heater 802 and which governs the power level ofthe roller heater 802 in response to that measured temperature. Thecontrol system may be of the proportional-integral-derivative (PID)variety or may use other control techniques as are known in the art.While roller 402 is being heated, roller 402 may be rotated, so as todistribute heat evenly over the surface of the roller, even if roller402 is not actually performing rolling of powder at that time.

[0245] The roller 402 may be heated to a temperature sufficient toprevent condensation of the evaporating solvent onto the roller. Anycondensable vapor carried in a noncondensable gas has a dew point orcondensation temperature that is a function of the vapor'sconcentration. In the case of an aqueous binder liquid, theconcentration of the vapor is the local humidity and the dew point isindicative of the local humidity. The roller 402 may be heated to atemperature above the dew point. In the case of an aqueous binderliquid, for typical operating conditions, a roller temperature of 35 C.is estimated to be appropriate. The roller 402 even may be heated to atemperature approximately equal to the temperature to which the surfaceof the second powder bed 302 is heated during interlayer drying, whichmight be a higher temperature than the dew point.

[0246] The roller 402 has so far been described as being heated by aheat source that is external to the roller, but it is also possible thata heat source could be placed inside the roller 402. For example, theroller 402 could be hollow and an electrical resistive heater could belocated in the interior of the roller 402. Similarly, the temperaturesensor 806 has been described as being external to roller 402, but itcould instead be placed inside the roller.

[0247] The use of a heated roller has been described herein inconnection with a three-dimensional printing machine 100 that usesinterlayer drying by means of a heating assembly 600. Interlayer dryingresults in accelerated evaporation of the solvent contained in thebinder liquid and means that in the absence of roller 402 heating, theroller may be one of the coolest components in the immediate vicinity ofthe second powder bed 302, which may facilitate condensation of solventvapor onto the roller. Thus, the situation of interlayer drying may makea heated roller 402 especially useful. Nevertheless, a heated roller 402may also be useful in other circumstances including ordinarythree-dimensional printers without the use of interlayer drying. Whileelectrical heat sources for heating the roller 402 have been describedherein, other heat sources are also possible as are known in the art.Heating could also be accomplished by directing heated air or other gasat the roller 402 from a source of heated gas. It would also be possibleto flow a heated liquid or gas through the roller 402 and therebyprovide heat, or to flow a heated liquid or gas through a device nearthe roller, which provides heat to the roller. It would also be possibleto pass an electric current through the roller 402 and generate heat byresistive dissipation. Radiant heaters may be enclosed in a glass orquartz tube as described here, but they do not need to be so enclosedand could instead be exposed. Another possible heat source is heatlamps, such as ordinary incandescent lamps or incandescent lamps thatare adapted to provide a significant amount of infrared radiation. Theroller 402 may have a temperature at the mid-point of its length that isdifferent from the temperature near its ends. If this is undesirable, orfor any other reason, it is possible to provide heat to the roller inamounts or fluxes which are different at different places along thelength of the roller 402. For example, if the ends of the roller 402 aretoo cold, it is possible to provide extra heat near the ends of theroller. Heat supply which is nonuniform along the length of the roller402 can be achieved by bunching together in closer proximity in desiredplaces any of the heating sources described herein, which may includevarying the spacing of resistive coils inside a quartz or glass tube ifthat is used. It is possible that the mountings or bearings of theroller 402 could be heated to minimize loss of heat from the roller toits ends or even to supply heat to the roller. The roller 402 may belonger than the build bed so that the temperature in the immediatevicinity of the roller ends does not directly affect the rolling itself.

[0248] Scrapers or brushes may be provided in contact with the roller atappropriate times and places to remove any powder which may stick to theroller despite these efforts or which may stick to the roller for anyother reasons. It is possible that even in the absence of considerationsabout condensation of vapors on the roller 402, heating the roller maybeneficially improve powder spreading by reducing the tendency of powderto stick to the roller due to reasons of electrostatic attraction orattractive Hamaker forces. It is possible to electrically ground theroller 402 or to maintain it at any desired electrical potential. Forelectrical grounding purposes or for heating by resistive dissipation,appropriate electrical contact with the roller 402 could be provided byslip rings, brushes or similar means.

[0249] Fluid Supply System

[0250]FIGS. 53 and 54 illustrate one particular fluid supply system 900that can be used with the microvalves described earlier. FIG. 53illustrates the fluid supply system receiving compressed gas, such asfrom a commercial or industrial compressed air source, at an inletfitting 902 and routing it to a mist remover 904 and a filter 906.Alternatively, other sources of gas could be used. The mist remover 904and the filter 906 are followed by a regulator 908, which may be aprecision regulator such as is available from the Watts RegulatorCompany, North Andover, Mass. Air or other gas which has passed throughregulator 908 proceeds through a filter 910(FIG. 54), optionally througha quick-disconnect fitting 912, and into a binder vessel 914. The bindervessel 914 (McMaster-Carr, Dayton, N.J.) may contain a top opening 916for loading binder liquid into the vessel, cleaning, etc. The bindervessel 914 may further comprise a dip tube 918 which communicates withnearly the lowest point of the interior of binder vessel 914, andconfigured to withdraw liquid under the influence of elevated pressurein an air space 920 in the upper portion of the binder vessel 914. Afitting 922 at the liquid outflow connection the binder vessel 914 mayalso be a quick-disconnect fitting. From there the liquid exiting thebinder vessel 914 proceeds through an in-line filter 924, and there to atee 926, one branch of which is coupled to a pressure transducer 928 andthe other branch of which goes to manifold 930 which divides the flowinto the appropriate number (shown as 8) individual flowpaths going toindividual dispensers. Manifold 930 also comprises a bubble release orbleed valve 932 to help in purging gas bubbles from the fluid system.

[0251] In-Line Flow Measurement by Differential Pressure and by BubbleMotion

[0252] It may be desirable to measure the flowrate of binder liquid asit is being dispensed through a particular dispenser as part of thethree-dimensional printing process. Some manufacturing processes mayhave tight requirements for delivered flowrate or at least forverification of delivered flowrate. This is particularly true in thecase of medical products, and particularly true if an active compound isdispensed contained in the binder liquid. Then, the accuracy of thecontent of the active compound is limited by the accuracy with which theflowrate of the binder liquid is known.

[0253] One way of measuring the flowrate of binder liquid as it isdelivered is to install a flowmeter in the fluid delivery line upstreamof the dispenser. In-line flowmetering at much larger scales offlowrates is quite common in industry. What is unusual about thisapplication is that the flowrates to be measured can be as small as tensof microliters per minute. One way in which flowrates at industrialscales of flowrates have been measured is by a differential pressure orobstruction type flowmeter. In such a flowmeter, flow is forced to flowthrough an obstruction or flow resistance element, which may be anorifice, a nozzle, a channel or array of channels having parallel walls,or other geometry. The pressure difference across the obstruction ismeasured, such as by pressure transducers. The measured pressuredifference indicates the flowrate, which is inferred through acalibration relation between flowrate and pressure drop. For use at verysmall flowrates, in order to obtain measurable pressure differences, thedimensions of such a device must be unusually small. For example, thedimensions of the channel or orifice or other form of flow obstructionmight be such as to be manufactured by silicon micromachining. Thepressure transducers used to measure the pressure difference can also bemanufactured by silicon micromachining and microfabrication techniques,possibly in the same chip as the flow resistance element. Suchflowmeters are commercially available from Seyonic S.A., Neuchatel,Switzerland. Information about flowrate can be recorded, for example foruse in verification that the part was built as intended. It is alsopossible to integrate, as a function of time, the signal that representsdispensed flowrate, and thereby obtain a delivered quantity of fluid.Such a flowmeter has the advantage that it can be used on a trulycontinuous basis and is truly in-line, always measuring the flow that isdelivered to the dispenser. Such a flowmeter can even measure reverseflow should that occur, in the sense that the direction of pressuredifference between the two transducers would reverse.

[0254] This type of system is further illustrated in FIG. 55. Fluid fromsource 802 flows through passageway 852 to flowmeter assembly 868 andthen on to dispenser 830. Within flowmeter assembly 868 is a flowresistance element 850. This is illustrated as being a plurality ofparallel paths or surfaces whose individual cross-sectional dimension issmaller than the dimension of passageway 852. However, other geometries,including orifices, are also possible.

[0255] Transducer 862 could be a differential pressure transducer, withthe pressure delivered to one side of its diaphragm being the pressureupstream of the flow resistance element 850 and the pressure deliveredto the other side of its diaphragm being the pressure downstream of theflow resistance element 850. Output from transducer 862 can be deliveredto a computer or other data acquisition system. Processing there caninclude linearization of the signal if the signal is nonlinearly relatedto flowrate. Alternatively, an individual pressure transducer could beused for each side of flow resistance element 850, with the outputs ofboth transducers being delivered to a computer or other data acquisitionsystem. Both the flow resistance element 850 and the pressuretransducer(s) 862 may be fabricated on a single chip of silicon 866. Thecross-sectional dimensions of the individual passageways in the flowresistance element 850 may be chosen to give a differential pressure,which is measurable with sufficient accuracy at the flowrates beingdispensed. For example, the cross-sectional dimensions of the individualpassageways in the flow resistance element 850, or of the flowresistance element's orifice if the flow resistance element is anorifice, may be roughly comparable to the diameter of the orificethrough which the liquid is dispensed at the dispenser 830.

[0256] Another method of making an in-line flowmeter, such as to measurethe dispensing of binder liquid, is based on measuring the movement of abubble in a transparent conduit. In general the liquid supply system maybe designed and operated so as to be substantially free of bubbles, butbubbles may be introduced into the system specifically for the purposeof indicating or measuring flow. A bubble provides an interface which isreadily identifiable and whose position may be measured. A bubble may beintroduced into the system upstream of the actual dispenser, and thenthe motion of the bubble may be observed, and then the bubbles may laterbe removed from the system after they have been used for flowmeasurement, before they reach the actual dispenser. Bubble removalprior to the actual dispenser is to avoid possible adverse effects onthe dispenser if bubbles reached the dispenser.

[0257] One particular bubble-based in-line flow measurement system forthree-dimensional printing is illustrated in FIG. 56. As would be truewith any in-line flowmeter, this type of measurement may be performed ata location which has substantially the same flowrate as occurs at thedispenser itself, i.e., in-line with the flow being measured and notsubject to any removal or addition of flow between the measurementlocation and the dispenser, while measurement is being performed. Thesystem may comprise a reservoir or pressurized source 802 of liquid thatmay be substantially bubble-free. From source 802 the liquid maycontinue on to a conduit 804 having a known cross-sectional area.Because the present application is concerned with measuring flowratesthat are small, the conduit 804 may be of small cross-sectionaldimension such as less than 1 mm diameter. The conduit 804 may be atleast partially transparent to permit visual or optical observation, andmay be made of polymeric material that is compatible with the liquidbeing carried. Along conduit 804 may be a bubble introduction means 810such as a hypodermic needle suitable to inject a suitable quantity ofgas into the liquid. The quantity of gas may be chosen so that thebubble thus formed occupies the entire cross-section of the conduit 804and has a length slightly longer than the conduit diameter, althoughthese characteristics are not absolutely necessary. The bubble may be ofair or could be of some other gas, such as helium, which is known tohave relatively low solubility in liquids.

[0258] The bubble advances when the liquid surrounding the bubbleadvances, and, if the bubble occupies the full cross-section of conduit804, there is essentially no passage of liquid from one side of thebubble to the other side. The edges of the bubble are readily observablethrough visual or optical means, and the position of a bubble edge as afunction of time indicates the dispensing of liquid. Measurements may bedone of either the leading edge or the trailing edge of the bubble 806.Suitable illumination 814 and a spatial reference scale 816 may beprovided. Observing means 826 may include appropriate cameras, lenses,and image acquisition systems, which may include digital algorithms fordetecting edges or contrast to indicate the edge of the bubble. Imageacquisition systems may be able to determine the position of the bubbleedge to a resolution governed by the pixel size on the image plane ofthe camera. As an example of the system according to the presentinvention, a flowrate of 8.64 microliters/minute (which may be obtainedby jetting drops of ethanol at 2 kHz through a 60 micron orifice)corresponds, using a tubing having an inside diameter of 0.57 mm, to abubble advance rate of 34 mm/minute. Increments of motion of 0.04 mm,corresponding to a dispensed volume of 0.01 microliter, are detectable.

[0259] As already discussed, in typical fluid dispensing systems, itwould generally not be desirable for the bubble 806 to be allowed totravel all the way through the system to the dispenser 830, because manytypes of dispensers (e.g., microvalve dispensers, piezoelectricdrop-on-demand dispensers) would be adversely affected by the presenceof gas bubbles. Accordingly, it would be desirable to remove the bubble806 after it has passed through the region used for tracking theposition and rate of advance of the bubble. This can be done bywithdrawing a portion of liquid containing the bubble 806, at a locationdownstream of the bubble-tracking region. As shown in FIG. 56, this canbe performed by bubble removal means 820 which may resemble the bubbleinjection means 810, except configured to operate so as to withdraw abubble and most likely some associated liquid. The bubble withdrawalmeans could instead be a valve. The inventors appreciate that otherbubble withdrawal or separation means could also be used, particularlythose known for use with intravenous delivery of liquids to patients.Alternatively, there could be provided a bubble collection region, whichprovides some space vertically above the liquid in conduit 804, suitableto collect some number of bubbles. However, for practical reasons itmight not be desirable to collect too large a volume of bubbles, becausesuch collected gas would be compressible and subject to volume changewhich might not be distinguishable from the exiting of flow through thedispenser. Thus, even a bubble collection region would have to beemptied occasionally. During those occasions when the bubble 806 andsome surrounding liquid are withdrawn from the system, it would bedifficult or impossible for this flow measurement system to determinethe flowrate, if any, through the dispenser. Accordingly, withdrawal ofa bubble may be performed in conjunction with expected interruptions inthe manufacturing process, when dispensing of binder is not beingperformed. For example, bubble withdrawal may be performed during timeswhen interlayer drying is performed, if that process step is in use, orduring deposition of a new layer of powder, or at similar non-printingtimes.

[0260] Information from observing means 826 may be integrated with othersoftware or hardware to yield an automatic calculation of the flowratebased on the position of the interface and the time. Either the bubbleinjection means 810 or the bubble withdrawal means 820 or both may beoperated automatically such as with programmed motion under the commandof a centralized computer 828. This computer or another computer mayalso calculate flowrates from time information and the position of thebubble. This information may be stored for record-keeping andverification purposes together with information about the portion of theprint job associated with the flowrate information. In addition, thecomputer may use the acquired information about bubble position todetermine when the bubble is near the end of the measurement region andneeds to be withdrawn from the liquid system.

[0261] For any of these flowmeters, what the flowmeter measures is thepassage of liquid through the flowmeter in an amount that issubstantially equal to the amount which is being dispensed at the sametime. This equivalence is of course aided by the fact that liquids areessentially incompressible in these circumstances. Also, it is helpfulto minimize the amount of elasticity or compliance of the boundary ofthe fluid region between the flowmeter and the dispenser (with thepossible exception of the case where a water hammer suppressor may beneeded). This can be achieved by locating the flowmeter relatively closeto the dispenser, such as on the printhead. However, other locations arealso possible. With sufficient resolution of the flowmeter, and forsuitably sized drops, at a sufficiently small dispense rate, theflowmeter may be able to measure the dispensing of an individual drop.For other circumstances, it may only be possible to measure the passageof some number of drops.

[0262] For any type of flowmeter, the flowrate information thus acquiredmay be stored in a computer or other memory for record-keeping andverification purposes. Flowrate information may be associated withinformation about the portion of the print job such a time and locationetc. of the dispenser when the flowrate was measured. This may be donefor multiple channels or dispensers. In addition, the flowrateinformation may be used in real time during the printing of a job. Forexample, the measured flowrate may be compared with a desired flowrate.If the flowrate is not what is desired, it may be possible to adjustcommand signals to the dispenser to make a correction. For example,microvalve dispensers vary the flowrate or drop size as a function ofthe waveform supplied to the microvalve. Piezoelectric dispensers alsocan vary the flowrate or drop size as a function of the waveform withwhich they are powered.

[0263] Similarly, it may be desirable to verify the delivery of a dropin particular locations where they were commanded. Detection of dropdelivery may not be sufficient to provide a highly accurate flowrate,just an indication that a drop in an approximate size range wasdelivered. One technique for doing so has been described in copendingand commonly assigned application U.S. Ser. No. 10/007,795, entitled “Amethod and apparatus for obtaining information about a dispensed fluid,such as using optical fiber to obtain diagnostic information about afluid at a printhead during printing, which is hereby incorporated byreference in its entirety. This technique can be used as part of themachine of the present invention.

[0264] Another U.S. patent application incorporated by reference in itsentirety is commonly assigned U.S. patent application Ser. No.09/991,556 entitled “Printing or dispensing a suspension such asthree-dimensional printing of dosage forms.” Another U.S. patentapplication incorporated by reference in its entirety is Method andapparatus for engineered regenerative biostructures such ashydroxyapatite substrates for bone healing applications, U.S. Ser. No.10/122,129.

[0265] Other U.S. patents incorporated by reference in their entiretyare commonly assigned U.S. Pat. Nos. 5,934,343; 6,213,168; and6,336,480. These describe apparatus and methods of presenting powder toa roller other than by a feed bed. It should be understood that in thepresent invention, in places where a feed bed is mentioned, other powderpresentation apparatus such as described in these patents could also beused.

[0266] Releasable gripping of the carrier plate 222, 322 could be doneelectromagnetically if suitable materials were used. Althoughdescription has focused on raster printing, it would also be possible toprint using vector printing or a combination of raster printing andvector printing.

[0267] All of the above U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet, areincorporated herein by reference, in their entirety.

[0268] From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

[0269] The above description of various illustrated embodiments of theinvention is not intended to be exhaustive or to limit the invention tothe precise form disclosed. While specific embodiments of, and examplesfor, the invention are described herein for illustrative purposes,various equivalent modifications are possible within the scope of theinvention, as those skilled in the relevant art will recognize. Theteachings provided herein of the invention can be applied to otherpurposes, other than the examples described above.

[0270] The various embodiments described above can be combined toprovide further embodiments.

[0271] For example, when the interlayer dryer features are used, it maybe helpful to also use the heated roller features, because condensationon an unheated roller might be higher. When the interlayer dryerfeatures are used, it may be helpful to have the ventilation system suchas is available with the enclosure. When the interlayer dryer featuresare used, it may be helpful to be able to rotate the roller during timeswhen rolling is not being performed, and the elevation of the rollerabove the guiderails may be helpful. The use of organic solvents maymake the enclosure and ventilation more necessary. In general, anyfeature can be combined with any other features in any combination.

[0272] Aspects of the invention can be modified, if necessary, to employthe process, apparatuses and concepts of the various patents,applications and publications described above to provide yet furtherembodiments of the invention.

[0273] These and other changes can be made to the invention in light ofthe above detailed description. In general, in the following claims, theterms used should not be construed to limit the invention to thespecific embodiments disclosed in the specification and the claims, butshould be construed to include all devices that operate under the claimsto provide an improved three-dimensional printing apparatus and method.Accordingly, the invention is not limited by the disclosure, but insteadthe scope of the invention is to be determined entirely by the followingclaims.

1. A three-dimensional printing machine comprising: a carriage mountedon the three-dimensional printing machine to controllably move in thedirection of a slow axis with respect to the three-dimensional printingmachine; an elongated encoder strip extending along the carriage in thedirection of a fast axis perpendicular to the slow axis, the encoderstrip being fixed with respect to the carriage; and a printhead coupledto the carriage to controllably move in the direction of the fast axiswith respect to the carriage and the three-dimensional printer, a readerbeing configured to read data from the encoder strip, the printheadbeing controlled to dispense fluid based on the data.
 2. Thethree-dimensional printing machine of claim 1 wherein the data comprisesthe detection of markings on the encoder strip and the printhead iscontrolled to dispense fluid based on counting the number of detectionsof markings since the last dispense or since some other event.
 3. Thethree-dimensional printing machine of claim 1 wherein the printheadcomprises one or more kinds of dispensers selected from the groupconsisting of microvalve dispensers, piezoelectric drop-on-demanddispensers, thermal drop-on-demand dispensers,continuous-jet-with-deflection dispensers, electrostatic deflectiondrop-on-demand dispensers, and other kinds of dispensers.
 4. Thethree-dimensional printing machine of claim 1, further comprising amotor and a screw drive configured to move the carriage with respect tothe three-dimensional printing machine.
 5. The three-dimensionalprinting machine of claim 1, further comprising a motor and a precisionscrew and ball drive configured to move the carriage with respect to thethree-dimensional printing machine.
 6. The three-dimensional printingmachine of claim 1, further comprising a linear motor for moving thecarriage and/or the printhead.
 7. The three-dimensional printing machineof claim 1 wherein the printhead is coupled to an air bearing which is,in turn, slidably mounted to the carriage to move along the carriagewith low friction.
 8. A three-dimensional printing machine comprising:rails mounted to the three-dimensional printing machine; bearingassemblies which ride on the rails, the bearing assemblies on at leastone of the rails being configured to deform laterally in response toexpansion or contraction of components of the three-dimensional printingmachine; a carriage movably mounted on the bearing assemblies tocontrollably move in the direction of a slow axis with respect to thethree-dimensional printing machine; and a printhead coupled to thecarriage to controllably move in the direction of the fast axis withrespect to the carriage and the three-dimensional printer.
 9. Aprinthead assembly for a three-dimensional printing machine, theprinthead assembly comprising: a body; and a plurality of dispensersindependently, removably coupled to the body by connectors such that onedispenser can be removed and replaced without adjusting others of theplurality of dispensers, wherein each connector has fins to transferheat away from the dispenser.
 10. The printhead assembly of claim 9,further comprising a chamber located upstream of the dispenser, thechamber having a first region configured to retain fluid for delivery tothe dispenser during operation and a second region at least partiallyadjacent to the first region, the second region being configured toretain a gas, and further comprising a diaphragm positioned between thefirst region and the second region to absorb pressure fluctuations inthe fluid.
 11. The printhead assembly of claim 10, comprising aplurality of chambers, each chamber being dedicated for use with aparticular dispenser.
 12. The printhead assembly of claim 10, comprisingmore than one dispenser, each dispenser being dedicated for use with aparticular fluid.
 13. The printhead assembly of claim 9 wherein eachdispenser has a nozzle or orifice through which the fluid is dispensedduring operation, each of the nozzles or orifices is independentlybonded to a respective nozzle/orifice plate, and each nozzle/orificeplate has a locating feature that mates with a complementary locatingfeature coupled to the body of the printhead to facilitate alignment ofthe dispenser.
 14. The printhead assembly of claim 9 wherein thedispensers are microvalve dispensers.
 15. The printhead assembly ofclaim 9 wherein the dispensers are piezoelectric dispensers.
 16. Theprinthead assembly of claim 9, comprising some dispensers dedicated foruse with a fluid which is a slurry containing powder particles andcomprising other dispensers dedicated for use with a fluid which iscapable of causing the powder particles to adhere to each other.
 17. Theprinthead assembly of claim 9, wherein at least some dispensers comprisea return path for fluid to exit from the dispenser in the return pathwithout being dispensed, and the printhead assembly provides space forthe return path of fluid.